diff --git a/javascript/src/stable.ts b/javascript/src/stable.ts
index e83b127f..ae26812b 100644
--- a/javascript/src/stable.ts
+++ b/javascript/src/stable.ts
@@ -517,6 +517,30 @@ export function loadIncremental<T>(
   return progressDocument(doc, heads, opts.patchCallback || state.patchCallback)
 }
 
+/**
+ * Create binary save data to be appended to a save file or fed into {@link loadIncremental}
+ *
+ * @typeParam T - The type of the value which is contained in the document.
+ *                Note that no validation is done to make sure this type is in
+ *                fact the type of the contained value so be a bit careful
+ *
+ * This function is useful for incrementally saving state.  The data can be appended to a
+ * automerge save file, or passed to a document replicating its state.
+ *
+ */
+export function saveIncremental<T>(doc: Doc<T>): Uint8Array {
+  const state = _state(doc)
+  if (state.heads) {
+    throw new RangeError(
+      "Attempting to change an out of date document - set at: " + _trace(doc)
+    )
+  }
+  if (_is_proxy(doc)) {
+    throw new RangeError("Calls to Automerge.change cannot be nested")
+  }
+  return state.handle.saveIncremental()
+}
+
 /**
  * Export the contents of a document to a compressed format
  *
diff --git a/javascript/src/types.ts b/javascript/src/types.ts
index beb5cf70..1166ccea 100644
--- a/javascript/src/types.ts
+++ b/javascript/src/types.ts
@@ -4,8 +4,8 @@ export { Counter } from "./counter"
 export { Int, Uint, Float64 } from "./numbers"
 
 import { Counter } from "./counter"
-import type { Patch } from "@automerge/automerge-wasm"
-export type { Patch } from "@automerge/automerge-wasm"
+import type { Patch, PatchInfo } from "@automerge/automerge-wasm"
+export type { Patch, Mark } from "@automerge/automerge-wasm"
 
 export type AutomergeValue =
   | ScalarValue
@@ -36,11 +36,9 @@ export type Doc<T> = { readonly [P in keyof T]: T[P] }
  * Callback which is called by various methods in this library to notify the
  * user of what changes have been made.
  * @param patch - A description of the changes made
- * @param before - The document before the change was made
- * @param after - The document after the change was made
+ * @param info - An object that has the "before" and "after" document state, and the "from" and "to" heads
  */
 export type PatchCallback<T> = (
   patches: Array<Patch>,
-  before: Doc<T>,
-  after: Doc<T>
+  info: PatchInfo<T>
 ) => void
diff --git a/javascript/src/unstable.ts b/javascript/src/unstable.ts
index 7c73afb9..9ce92698 100644
--- a/javascript/src/unstable.ts
+++ b/javascript/src/unstable.ts
@@ -44,11 +44,12 @@ export {
   Float64,
   type Patch,
   type PatchCallback,
+  type Mark,
   type AutomergeValue,
   type ScalarValue,
 } from "./unstable_types"
 
-import type { PatchCallback } from "./stable"
+import type { ScalarValue, Mark, PatchCallback } from "./stable"
 
 import { type UnstableConflicts as Conflicts } from "./conflicts"
 import { unstableConflictAt } from "./conflicts"
@@ -197,7 +198,11 @@ export function load<T>(
 ): Doc<T> {
   const opts = importOpts(_opts)
   opts.enableTextV2 = true
-  return stable.load(data, opts)
+  if (opts.patchCallback) {
+    return stable.loadIncremental(stable.init(opts), data)
+  } else {
+    return stable.load(data, opts)
+  }
 }
 
 function importOpts<T>(
@@ -233,6 +238,66 @@ export function splice<T>(
   }
 }
 
+export function mark<T>(
+  doc: Doc<T>,
+  prop: stable.Prop,
+  name: string,
+  range: string,
+  value: ScalarValue
+) {
+  if (!_is_proxy(doc)) {
+    throw new RangeError("object cannot be modified outside of a change block")
+  }
+  const state = _state(doc, false)
+  const objectId = _obj(doc)
+  if (!objectId) {
+    throw new RangeError("invalid object for mark")
+  }
+  const obj = `${objectId}/${prop}`
+  try {
+    return state.handle.mark(obj, range, name, value)
+  } catch (e) {
+    throw new RangeError(`Cannot mark: ${e}`)
+  }
+}
+
+export function unmark<T>(
+  doc: Doc<T>,
+  prop: stable.Prop,
+  name: string,
+  start: number,
+  end: number
+) {
+  if (!_is_proxy(doc)) {
+    throw new RangeError("object cannot be modified outside of a change block")
+  }
+  const state = _state(doc, false)
+  const objectId = _obj(doc)
+  if (!objectId) {
+    throw new RangeError("invalid object for unmark")
+  }
+  const obj = `${objectId}/${prop}`
+  try {
+    return state.handle.unmark(obj, name, start, end)
+  } catch (e) {
+    throw new RangeError(`Cannot unmark: ${e}`)
+  }
+}
+
+export function marks<T>(doc: Doc<T>, prop: stable.Prop): Mark[] {
+  const state = _state(doc, false)
+  const objectId = _obj(doc)
+  if (!objectId) {
+    throw new RangeError("invalid object for unmark")
+  }
+  const obj = `${objectId}/${prop}`
+  try {
+    return state.handle.marks(obj)
+  } catch (e) {
+    throw new RangeError(`Cannot call marks(): ${e}`)
+  }
+}
+
 /**
  * Get the conflicts associated with a property
  *
diff --git a/javascript/src/unstable_types.ts b/javascript/src/unstable_types.ts
index 071e2cc4..a89fc8e3 100644
--- a/javascript/src/unstable_types.ts
+++ b/javascript/src/unstable_types.ts
@@ -8,6 +8,7 @@ export {
   Float64,
   type Patch,
   type PatchCallback,
+  type Mark,
 } from "./types"
 
 import { RawString } from "./raw_string"
diff --git a/javascript/test/legacy_tests.ts b/javascript/test/legacy_tests.ts
index 8c2e552e..a078e30b 100644
--- a/javascript/test/legacy_tests.ts
+++ b/javascript/test/legacy_tests.ts
@@ -340,8 +340,7 @@ describe("Automerge", () => {
         const s2 = Automerge.change(
           s1,
           {
-            patchCallback: (patches, before, after) =>
-              callbacks.push({ patches, before, after }),
+            patchCallback: (patches, info) => callbacks.push({ patches, info }),
           },
           doc => {
             doc.birds = ["Goldfinch"]
@@ -363,8 +362,8 @@ describe("Automerge", () => {
           path: ["birds", 0, 0],
           value: "Goldfinch",
         })
-        assert.strictEqual(callbacks[0].before, s1)
-        assert.strictEqual(callbacks[0].after, s2)
+        assert.strictEqual(callbacks[0].info.before, s1)
+        assert.strictEqual(callbacks[0].info.after, s2)
       })
 
       it("should call a patchCallback set up on document initialisation", () => {
@@ -374,8 +373,7 @@ describe("Automerge", () => {
           after: Automerge.Doc<any>
         }> = []
         s1 = Automerge.init({
-          patchCallback: (patches, before, after) =>
-            callbacks.push({ patches, before, after }),
+          patchCallback: (patches, info) => callbacks.push({ patches, info }),
         })
         const s2 = Automerge.change(s1, doc => (doc.bird = "Goldfinch"))
         assert.strictEqual(callbacks.length, 1)
@@ -389,8 +387,8 @@ describe("Automerge", () => {
           path: ["bird", 0],
           value: "Goldfinch",
         })
-        assert.strictEqual(callbacks[0].before, s1)
-        assert.strictEqual(callbacks[0].after, s2)
+        assert.strictEqual(callbacks[0].info.before, s1)
+        assert.strictEqual(callbacks[0].info.after, s2)
       })
     })
 
@@ -1570,7 +1568,7 @@ describe("Automerge", () => {
       assert.deepStrictEqual(doc, { list: expected })
     })
 
-    it.skip("should call patchCallback if supplied to load", () => {
+    it("should call patchCallback if supplied to load", () => {
       const s1 = Automerge.change(
         Automerge.init<any>(),
         doc => (doc.birds = ["Goldfinch"])
@@ -1579,40 +1577,19 @@ describe("Automerge", () => {
       const callbacks: Array<any> = [],
         actor = Automerge.getActorId(s1)
       const reloaded = Automerge.load<any>(Automerge.save(s2), {
-        patchCallback(patch, before, after) {
-          callbacks.push({ patch, before, after })
+        patchCallback(patches, opts) {
+          callbacks.push({ patches, opts })
         },
       })
       assert.strictEqual(callbacks.length, 1)
-      assert.deepStrictEqual(callbacks[0].patch, {
-        maxOp: 3,
-        deps: [decodeChange(Automerge.getAllChanges(s2)[1]).hash],
-        clock: { [actor]: 2 },
-        pendingChanges: 0,
-        diffs: {
-          objectId: "_root",
-          type: "map",
-          props: {
-            birds: {
-              [`1@${actor}`]: {
-                objectId: `1@${actor}`,
-                type: "list",
-                edits: [
-                  {
-                    action: "multi-insert",
-                    index: 0,
-                    elemId: `2@${actor}`,
-                    values: ["Goldfinch", "Chaffinch"],
-                  },
-                ],
-              },
-            },
-          },
-        },
-      })
-      assert.deepStrictEqual(callbacks[0].before, {})
-      assert.strictEqual(callbacks[0].after, reloaded)
-      assert.strictEqual(callbacks[0].local, false)
+      assert.deepStrictEqual(callbacks[0].patches, [
+        { action: "put", path: ["birds"], value: [] },
+        { action: "insert", path: ["birds", 0], values: ["", ""] },
+        { action: "splice", path: ["birds", 0, 0], value: "Goldfinch" },
+        { action: "splice", path: ["birds", 1, 0], value: "Chaffinch" },
+      ])
+      assert.deepStrictEqual(callbacks[0].opts.before, {})
+      assert.strictEqual(callbacks[0].opts.after, reloaded)
     })
   })
 
@@ -1812,8 +1789,8 @@ describe("Automerge", () => {
         before,
         Automerge.getAllChanges(s1),
         {
-          patchCallback(patch, before, after) {
-            callbacks.push({ patch, before, after })
+          patchCallback(patch, info) {
+            callbacks.push({ patch, info })
           },
         }
       )
@@ -1833,8 +1810,8 @@ describe("Automerge", () => {
         path: ["birds", 0, 0],
         value: "Goldfinch",
       })
-      assert.strictEqual(callbacks[0].before, before)
-      assert.strictEqual(callbacks[0].after, after)
+      assert.strictEqual(callbacks[0].info.before, before)
+      assert.strictEqual(callbacks[0].info.after, after)
     })
 
     it("should merge multiple applied changes into one patch", () => {
diff --git a/javascript/test/marks.ts b/javascript/test/marks.ts
new file mode 100644
index 00000000..538c2db1
--- /dev/null
+++ b/javascript/test/marks.ts
@@ -0,0 +1,63 @@
+import * as assert from "assert"
+import { unstable as Automerge } from "../src"
+import * as WASM from "@automerge/automerge-wasm"
+
+describe("Automerge", () => {
+  describe("marks", () => {
+    it("should allow marks that can be seen in patches", () => {
+      let callbacks = []
+      let doc1 = Automerge.init({
+        patchCallback: (patches, info) => callbacks.push(patches),
+      })
+      doc1 = Automerge.change(doc1, d => {
+        d.x = "the quick fox jumps over the lazy dog"
+      })
+      doc1 = Automerge.change(doc1, d => {
+        Automerge.mark(d, "x", "font-weight", "[5..10]", "bold")
+      })
+
+      doc1 = Automerge.change(doc1, d => {
+        Automerge.unmark(d, "x", "font-weight", 7, 9)
+      })
+
+      assert.deepStrictEqual(callbacks[1], [
+        {
+          action: "mark",
+          path: ["x"],
+          marks: [{ name: "font-weight", start: 5, end: 10, value: "bold" }],
+        },
+      ])
+
+      assert.deepStrictEqual(callbacks[2], [
+        {
+          action: "unmark",
+          path: ["x"],
+          name: "font-weight",
+          start: 7,
+          end: 9,
+        },
+      ])
+
+      callbacks = []
+
+      let doc2 = Automerge.init({
+        patchCallback: (patches, info) => callbacks.push(patches),
+      })
+      doc2 = Automerge.loadIncremental(doc2, Automerge.save(doc1))
+
+      assert.deepStrictEqual(callbacks[0][2], {
+        action: "mark",
+        path: ["x"],
+        marks: [
+          { name: "font-weight", start: 5, end: 7, value: "bold" },
+          { name: "font-weight", start: 9, end: 10, value: "bold" },
+        ],
+      })
+
+      assert.deepStrictEqual(Automerge.marks(doc2, "x"), [
+        { name: "font-weight", value: "bold", start: 5, end: 7 },
+        { name: "font-weight", value: "bold", start: 9, end: 10 },
+      ])
+    })
+  })
+})
diff --git a/rust/automerge-wasm/index.d.ts b/rust/automerge-wasm/index.d.ts
index be12e4c1..4fb1c0c1 100644
--- a/rust/automerge-wasm/index.d.ts
+++ b/rust/automerge-wasm/index.d.ts
@@ -94,7 +94,7 @@ export type Op = {
   pred: string[],
 }
 
-export type Patch =  PutPatch | DelPatch | SpliceTextPatch | IncPatch | InsertPatch;
+export type Patch =  PutPatch | DelPatch | SpliceTextPatch | IncPatch | InsertPatch | MarkPatch | UnmarkPatch;
 
 export type PutPatch = {
   action: 'put'
@@ -103,6 +103,20 @@ export type PutPatch = {
   conflict: boolean
 }
 
+export type MarkPatch = {
+  action: 'mark'
+  path: Prop[],
+  marks: Mark[]
+}
+
+export type UnmarkPatch = {
+  action: 'unmark'
+  path: Prop[],
+  name: string,
+  start: number,
+  end: number
+}
+
 export type IncPatch = {
   action: 'inc'
   path: Prop[],
@@ -127,6 +141,13 @@ export type InsertPatch = {
   values: Value[],
 }
 
+export type Mark = {
+  name: string,
+  value: Value,
+  start: number,
+  end: number,
+}
+
 export function encodeChange(change: ChangeToEncode): Change;
 export function create(text_v2: boolean, actor?: Actor): Automerge;
 export function load(data: Uint8Array, text_v2: boolean, actor?: Actor): Automerge;
@@ -165,6 +186,11 @@ export class Automerge {
   increment(obj: ObjID, prop: Prop, value: number): void;
   delete(obj: ObjID, prop: Prop): void;
 
+  // marks
+  mark(obj: ObjID, name: string, range: string, value: Value, datatype?: Datatype): void;
+  unmark(obj: ObjID, name: string, start: number, end: number): void;
+  marks(obj: ObjID, heads?: Heads): Mark[];
+
   // returns a single value - if there is a conflict return the winner
   get(obj: ObjID, prop: Prop, heads?: Heads): Value | undefined;
   getWithType(obj: ObjID, prop: Prop, heads?: Heads): FullValue | null;
@@ -217,7 +243,14 @@ export class Automerge {
   dump(): void;
 
   // experimental api can go here
-  applyPatches<Doc>(obj: Doc, meta?: unknown, callback?: (patch: Array<Patch>, before: Doc, after: Doc) => void): Doc;
+  applyPatches<Doc>(obj: Doc, meta?: unknown, callback?: (patch: Array<Patch>, info: PatchInfo<Doc>) => void): Doc;
+}
+
+export interface PatchInfo<T> {
+  before: T,
+  after: T,
+  from: Heads,
+  to: Heads,
 }
 
 export interface JsSyncState {
@@ -236,3 +269,4 @@ export class SyncState {
   sentHashes: Heads;
   readonly sharedHeads: Heads;
 }
+
diff --git a/rust/automerge-wasm/package.json b/rust/automerge-wasm/package.json
index 80b39fd4..61209706 100644
--- a/rust/automerge-wasm/package.json
+++ b/rust/automerge-wasm/package.json
@@ -30,6 +30,7 @@
   "scripts": {
     "lint": "eslint test/*.ts index.d.ts",
     "debug": "cross-env PROFILE=dev TARGET_DIR=debug yarn buildall",
+    "dev": "cross-env PROFILE=dev TARGET_DIR=debug FEATURES='' TARGET=nodejs yarn target",
     "build": "cross-env PROFILE=dev TARGET_DIR=debug FEATURES='' yarn buildall",
     "release": "cross-env PROFILE=release TARGET_DIR=release yarn buildall",
     "buildall": "cross-env TARGET=nodejs yarn target && cross-env TARGET=bundler yarn target && cross-env TARGET=deno yarn target",
@@ -42,6 +43,7 @@
   "devDependencies": {
     "@types/mocha": "^10.0.1",
     "@types/node": "^18.11.13",
+    "@types/uuid": "^9.0.1",
     "@typescript-eslint/eslint-plugin": "^5.46.0",
     "@typescript-eslint/parser": "^5.46.0",
     "cross-env": "^7.0.3",
@@ -51,7 +53,8 @@
     "pako": "^2.1.0",
     "rimraf": "^3.0.2",
     "ts-mocha": "^10.0.0",
-    "typescript": "^4.9.4"
+    "typescript": "^4.9.4",
+    "uuid": "^9.0.0"
   },
   "exports": {
     "browser": "./bundler/automerge_wasm.js",
diff --git a/rust/automerge-wasm/src/interop.rs b/rust/automerge-wasm/src/interop.rs
index 1546ff10..fbf252f3 100644
--- a/rust/automerge-wasm/src/interop.rs
+++ b/rust/automerge-wasm/src/interop.rs
@@ -12,7 +12,7 @@ use std::fmt::Display;
 use wasm_bindgen::prelude::*;
 use wasm_bindgen::JsCast;
 
-use crate::{observer::Patch, ObjId, Value};
+use am::{ObjId, Patch, PatchAction, Value};
 
 const RAW_DATA_SYMBOL: &str = "_am_raw_value_";
 const DATATYPE_SYMBOL: &str = "_am_datatype_";
@@ -28,6 +28,12 @@ impl From<AR> for JsValue {
     }
 }
 
+impl From<AR> for Array {
+    fn from(ar: AR) -> Self {
+        ar.0
+    }
+}
+
 impl From<JS> for JsValue {
     fn from(js: JS) -> Self {
         js.0
@@ -334,11 +340,20 @@ impl TryFrom<JS> for am::sync::Message {
     }
 }
 
+impl From<Vec<ChangeHash>> for AR {
+    fn from(values: Vec<ChangeHash>) -> Self {
+        AR(values
+            .iter()
+            .map(|h| JsValue::from_str(&h.to_string()))
+            .collect())
+    }
+}
+
 impl From<&[ChangeHash]> for AR {
     fn from(value: &[ChangeHash]) -> Self {
         AR(value
             .iter()
-            .map(|h| JsValue::from_str(&hex::encode(h.0)))
+            .map(|h| JsValue::from_str(&h.to_string()))
             .collect())
     }
 }
@@ -746,13 +761,13 @@ impl Automerge {
     pub(crate) fn apply_patch_to_array(
         &self,
         array: &Object,
-        patch: &Patch,
+        patch: &Patch<u16>,
         meta: &JsValue,
         exposed: &mut HashSet<ObjId>,
     ) -> Result<Object, error::ApplyPatch> {
         let result = Array::from(array); // shallow copy
-        match patch {
-            Patch::PutSeq {
+        match &patch.action {
+            PatchAction::PutSeq {
                 index,
                 value,
                 expose,
@@ -768,13 +783,13 @@ impl Automerge {
                 }
                 Ok(result.into())
             }
-            Patch::DeleteSeq { index, length, .. } => {
+            PatchAction::DeleteSeq { index, length, .. } => {
                 Ok(self.sub_splice(result, *index, *length, vec![], meta)?)
             }
-            Patch::Insert { index, values, .. } => {
+            PatchAction::Insert { index, values, .. } => {
                 Ok(self.sub_splice(result, *index, 0, values, meta)?)
             }
-            Patch::Increment { prop, value, .. } => {
+            PatchAction::Increment { prop, value, .. } => {
                 if let Prop::Seq(index) = prop {
                     let index = *index as f64;
                     let old_val = js_get(&result, index)?.0;
@@ -795,9 +810,9 @@ impl Automerge {
                     Err(error::ApplyPatch::IncrementKeyInSeq)
                 }
             }
-            Patch::DeleteMap { .. } => Err(error::ApplyPatch::DeleteKeyFromSeq),
-            Patch::PutMap { .. } => Err(error::ApplyPatch::PutKeyInSeq),
-            Patch::SpliceText { index, value, .. } => {
+            PatchAction::DeleteMap { .. } => Err(error::ApplyPatch::DeleteKeyFromSeq),
+            PatchAction::PutMap { .. } => Err(error::ApplyPatch::PutKeyInSeq),
+            PatchAction::SpliceText { index, value, .. } => {
                 match self.text_rep {
                     TextRepresentation::String => Err(error::ApplyPatch::SpliceTextInSeq),
                     TextRepresentation::Array => {
@@ -819,19 +834,20 @@ impl Automerge {
                     }
                 }
             }
+            PatchAction::Mark { .. } | PatchAction::Unmark { .. } => Ok(result.into()),
         }
     }
 
     pub(crate) fn apply_patch_to_map(
         &self,
         map: &Object,
-        patch: &Patch,
+        patch: &Patch<u16>,
         meta: &JsValue,
         exposed: &mut HashSet<ObjId>,
     ) -> Result<Object, error::ApplyPatch> {
         let result = Object::assign(&Object::new(), map); // shallow copy
-        match patch {
-            Patch::PutMap {
+        match &patch.action {
+            PatchAction::PutMap {
                 key, value, expose, ..
             } => {
                 if *expose && value.0.is_object() {
@@ -844,7 +860,7 @@ impl Automerge {
                 }
                 Ok(result)
             }
-            Patch::DeleteMap { key, .. } => {
+            PatchAction::DeleteMap { key, .. } => {
                 Reflect::delete_property(&result, &key.into()).map_err(|e| {
                     error::Export::Delete {
                         prop: key.to_string(),
@@ -853,7 +869,7 @@ impl Automerge {
                 })?;
                 Ok(result)
             }
-            Patch::Increment { prop, value, .. } => {
+            PatchAction::Increment { prop, value, .. } => {
                 if let Prop::Map(key) = prop {
                     let old_val = js_get(&result, key)?.0;
                     let old_val = self.unwrap_scalar(old_val)?;
@@ -873,27 +889,30 @@ impl Automerge {
                     Err(error::ApplyPatch::IncrementIndexInMap)
                 }
             }
-            Patch::Insert { .. } => Err(error::ApplyPatch::InsertInMap),
-            Patch::DeleteSeq { .. } => Err(error::ApplyPatch::SpliceInMap),
-            //Patch::SpliceText { .. } => Err(to_js_err("cannot Splice into map")),
-            Patch::SpliceText { .. } => Err(error::ApplyPatch::SpliceTextInMap),
-            Patch::PutSeq { .. } => Err(error::ApplyPatch::PutIdxInMap),
+            PatchAction::Insert { .. } => Err(error::ApplyPatch::InsertInMap),
+            PatchAction::DeleteSeq { .. } => Err(error::ApplyPatch::SpliceInMap),
+            //PatchAction::SpliceText { .. } => Err(to_js_err("cannot Splice into map")),
+            PatchAction::SpliceText { .. } => Err(error::ApplyPatch::SpliceTextInMap),
+            PatchAction::PutSeq { .. } => Err(error::ApplyPatch::PutIdxInMap),
+            PatchAction::Mark { .. } | PatchAction::Unmark { .. } => {
+                Err(error::ApplyPatch::MarkInMap)
+            }
         }
     }
 
     pub(crate) fn apply_patch(
         &self,
         obj: Object,
-        patch: &Patch,
+        patch: &Patch<u16>,
         depth: usize,
         meta: &JsValue,
         exposed: &mut HashSet<ObjId>,
     ) -> Result<Object, error::ApplyPatch> {
         let (inner, datatype, id) = self.unwrap_object(&obj)?;
-        let prop = patch.path().get(depth).map(|p| prop_to_js(&p.1));
+        let prop = patch.path.get(depth).map(|p| prop_to_js(&p.1));
         let result = if let Some(prop) = prop {
             let subval = js_get(&inner, &prop)?.0;
-            if subval.is_string() && patch.path().len() - 1 == depth {
+            if subval.is_string() && patch.path.len() - 1 == depth {
                 if let Ok(s) = subval.dyn_into::<JsString>() {
                     let new_value = self.apply_patch_to_text(&s, patch)?;
                     let result = shallow_copy(&inner);
@@ -914,12 +933,12 @@ impl Automerge {
                 return Ok(obj);
             }
         } else if Array::is_array(&inner) {
-            if &id == patch.obj() {
+            if id == patch.obj {
                 self.apply_patch_to_array(&inner, patch, meta, exposed)
             } else {
                 Ok(Array::from(&inner).into())
             }
-        } else if &id == patch.obj() {
+        } else if id == patch.obj {
             self.apply_patch_to_map(&inner, patch, meta, exposed)
         } else {
             Ok(Object::assign(&Object::new(), &inner))
@@ -932,17 +951,17 @@ impl Automerge {
     fn apply_patch_to_text(
         &self,
         string: &JsString,
-        patch: &Patch,
+        patch: &Patch<u16>,
     ) -> Result<JsValue, error::ApplyPatch> {
-        match patch {
-            Patch::DeleteSeq { index, length, .. } => {
+        match &patch.action {
+            PatchAction::DeleteSeq { index, length, .. } => {
                 let index = *index as u32;
                 let before = string.slice(0, index);
                 let after = string.slice(index + *length as u32, string.length());
                 let result = before.concat(&after);
                 Ok(result.into())
             }
-            Patch::SpliceText { index, value, .. } => {
+            PatchAction::SpliceText { index, value, .. } => {
                 let index = *index as u32;
                 let length = string.length();
                 let before = string.slice(0, index);
@@ -1205,6 +1224,148 @@ fn set_hidden_value<V: Into<JsValue>>(
     Ok(())
 }
 
+pub(crate) struct JsPatch(pub(crate) Patch<u16>);
+
+fn export_path(path: &[(ObjId, Prop)], end: &Prop) -> Array {
+    let result = Array::new();
+    for p in path {
+        result.push(&prop_to_js(&p.1));
+    }
+    result.push(&prop_to_js(end));
+    result
+}
+
+fn export_just_path(path: &[(ObjId, Prop)]) -> Array {
+    let result = Array::new();
+    for p in path {
+        result.push(&prop_to_js(&p.1));
+    }
+    result
+}
+
+impl TryFrom<JsPatch> for JsValue {
+    type Error = error::Export;
+
+    fn try_from(p: JsPatch) -> Result<Self, Self::Error> {
+        let result = Object::new();
+        let path = &p.0.path;
+        match p.0.action {
+            PatchAction::PutMap { key, value, .. } => {
+                js_set(&result, "action", "put")?;
+                js_set(
+                    &result,
+                    "path",
+                    export_path(path.as_slice(), &Prop::Map(key)),
+                )?;
+                js_set(
+                    &result,
+                    "value",
+                    alloc(&value.0, TextRepresentation::String).1,
+                )?;
+                Ok(result.into())
+            }
+            PatchAction::PutSeq { index, value, .. } => {
+                js_set(&result, "action", "put")?;
+                js_set(
+                    &result,
+                    "path",
+                    export_path(path.as_slice(), &Prop::Seq(index)),
+                )?;
+                js_set(
+                    &result,
+                    "value",
+                    alloc(&value.0, TextRepresentation::String).1,
+                )?;
+                Ok(result.into())
+            }
+            PatchAction::Insert { index, values, .. } => {
+                js_set(&result, "action", "insert")?;
+                js_set(
+                    &result,
+                    "path",
+                    export_path(path.as_slice(), &Prop::Seq(index)),
+                )?;
+                js_set(
+                    &result,
+                    "values",
+                    values
+                        .iter()
+                        .map(|v| alloc(&v.0, TextRepresentation::String).1)
+                        .collect::<Array>(),
+                )?;
+                Ok(result.into())
+            }
+            PatchAction::SpliceText { index, value, .. } => {
+                js_set(&result, "action", "splice")?;
+                js_set(
+                    &result,
+                    "path",
+                    export_path(path.as_slice(), &Prop::Seq(index)),
+                )?;
+                let bytes: Vec<u16> = value.iter().cloned().collect();
+                js_set(&result, "value", String::from_utf16_lossy(bytes.as_slice()))?;
+                Ok(result.into())
+            }
+            PatchAction::Increment { prop, value, .. } => {
+                js_set(&result, "action", "inc")?;
+                js_set(&result, "path", export_path(path.as_slice(), &prop))?;
+                js_set(&result, "value", &JsValue::from_f64(value as f64))?;
+                Ok(result.into())
+            }
+            PatchAction::DeleteMap { key, .. } => {
+                js_set(&result, "action", "del")?;
+                js_set(
+                    &result,
+                    "path",
+                    export_path(path.as_slice(), &Prop::Map(key)),
+                )?;
+                Ok(result.into())
+            }
+            PatchAction::DeleteSeq { index, length, .. } => {
+                js_set(&result, "action", "del")?;
+                js_set(
+                    &result,
+                    "path",
+                    export_path(path.as_slice(), &Prop::Seq(index)),
+                )?;
+                if length > 1 {
+                    js_set(&result, "length", length)?;
+                }
+                Ok(result.into())
+            }
+            PatchAction::Mark { marks, .. } => {
+                js_set(&result, "action", "mark")?;
+                js_set(&result, "path", export_just_path(path.as_slice()))?;
+                let marks_array = Array::new();
+                for m in marks.iter() {
+                    let mark = Object::new();
+                    js_set(&mark, "name", m.name())?;
+                    js_set(
+                        &mark,
+                        "value",
+                        &alloc(&m.value().into(), TextRepresentation::String).1,
+                    )?;
+                    js_set(&mark, "start", m.start as i32)?;
+                    js_set(&mark, "end", m.end as i32)?;
+                    marks_array.push(&mark);
+                }
+                js_set(&result, "marks", marks_array)?;
+                Ok(result.into())
+            }
+            PatchAction::Unmark {
+                name, start, end, ..
+            } => {
+                js_set(&result, "action", "unmark")?;
+                js_set(&result, "path", export_just_path(path.as_slice()))?;
+                js_set(&result, "name", name)?;
+                js_set(&result, "start", start as i32)?;
+                js_set(&result, "end", end as i32)?;
+                Ok(result.into())
+            }
+        }
+    }
+}
+
 fn shallow_copy(obj: &Object) -> Object {
     if Array::is_array(obj) {
         Array::from(obj).into()
@@ -1406,6 +1567,8 @@ pub(crate) mod error {
         SpliceTextInMap,
         #[error("cannot put a seq index in a map")]
         PutIdxInMap,
+        #[error("cannot mark a span in a map")]
+        MarkInMap,
         #[error(transparent)]
         GetProp(#[from] GetProp),
         #[error(transparent)]
diff --git a/rust/automerge-wasm/src/lib.rs b/rust/automerge-wasm/src/lib.rs
index 09072ca7..37ef737b 100644
--- a/rust/automerge-wasm/src/lib.rs
+++ b/rust/automerge-wasm/src/lib.rs
@@ -29,8 +29,10 @@ use am::transaction::CommitOptions;
 use am::transaction::{Observed, Transactable, UnObserved};
 use am::ScalarValue;
 use automerge as am;
-use automerge::{sync::SyncDoc, Change, ObjId, Prop, ReadDoc, TextEncoding, Value, ROOT};
+use automerge::{sync::SyncDoc, Change, Prop, ReadDoc, TextEncoding, Value, ROOT};
+use automerge::{ToggleObserver, VecOpObserver16};
 use js_sys::{Array, Function, Object, Uint8Array};
+use regex::Regex;
 use serde::ser::Serialize;
 use std::borrow::Cow;
 use std::collections::HashMap;
@@ -40,13 +42,9 @@ use wasm_bindgen::prelude::*;
 use wasm_bindgen::JsCast;
 
 mod interop;
-mod observer;
-mod sequence_tree;
 mod sync;
 mod value;
 
-use observer::Observer;
-
 use interop::{alloc, get_heads, import_obj, js_set, to_js_err, to_prop, AR, JS};
 use sync::SyncState;
 use value::Datatype;
@@ -60,7 +58,7 @@ macro_rules! log {
     };
 }
 
-type AutoCommit = am::AutoCommitWithObs<Observed<Observer>>;
+type AutoCommit = am::AutoCommitWithObs<Observed<ToggleObserver<VecOpObserver16>>>;
 
 #[cfg(feature = "wee_alloc")]
 #[global_allocator]
@@ -82,6 +80,15 @@ impl std::default::Default for TextRepresentation {
     }
 }
 
+impl From<TextRepresentation> for am::op_observer::TextRepresentation {
+    fn from(tr: TextRepresentation) -> Self {
+        match tr {
+            TextRepresentation::Array => am::op_observer::TextRepresentation::Array,
+            TextRepresentation::String => am::op_observer::TextRepresentation::String,
+        }
+    }
+}
+
 #[wasm_bindgen]
 #[derive(Debug)]
 pub struct Automerge {
@@ -97,7 +104,10 @@ impl Automerge {
         actor: Option<String>,
         text_rep: TextRepresentation,
     ) -> Result<Automerge, error::BadActorId> {
-        let mut doc = AutoCommit::default().with_encoding(TextEncoding::Utf16);
+        let mut doc = AutoCommit::default()
+            .with_observer(ToggleObserver::default().with_text_rep(text_rep.into()))
+            .with_encoding(TextEncoding::Utf16);
+        doc.observer().set_text_rep(text_rep.into());
         if let Some(a) = actor {
             let a = automerge::ActorId::from(hex::decode(a)?.to_vec());
             doc.set_actor(a);
@@ -545,8 +555,9 @@ impl Automerge {
         let enable = enable
             .as_bool()
             .ok_or_else(|| to_js_err("must pass a bool to enablePatches"))?;
-        let old_enabled = self.doc.observer().enable(enable);
-        self.doc.observer().set_text_rep(self.text_rep);
+        let heads = self.doc.get_heads();
+        let old_enabled = self.doc.observer().enable(enable, heads);
+        self.doc.observer().set_text_rep(self.text_rep.into());
         Ok(old_enabled.into())
     }
 
@@ -571,11 +582,12 @@ impl Automerge {
         object: JsValue,
         meta: JsValue,
         callback: JsValue,
-    ) -> Result<JsValue, error::ApplyPatch> {
+    ) -> Result<JsValue, JsValue> {
         let mut object = object
             .dyn_into::<Object>()
             .map_err(|_| error::ApplyPatch::NotObjectd)?;
-        let patches = self.doc.observer().take_patches();
+        let end_heads = self.doc.get_heads();
+        let (patches, begin_heads) = self.doc.observer().take_patches(end_heads.clone());
         let callback = callback.dyn_into::<Function>().ok();
 
         // even if there are no patches we may need to update the meta object
@@ -594,19 +606,24 @@ impl Automerge {
             object = self.apply_patch(object, p, 0, &meta, &mut exposed)?;
         }
 
+        self.finalize_exposed(&object, exposed, &meta)?;
+
         if let Some(c) = &callback {
             if !patches.is_empty() {
                 let patches: Array = patches
                     .into_iter()
+                    .map(interop::JsPatch)
                     .map(JsValue::try_from)
                     .collect::<Result<_, _>>()?;
-                c.call3(&JsValue::undefined(), &patches.into(), &before, &object)
-                    .map_err(error::ApplyPatch::PatchCallback)?;
+                let info = Object::new();
+                js_set(&info, "before", &before)?;
+                js_set(&info, "after", &object)?;
+                js_set(&info, "from", AR::from(begin_heads))?;
+                js_set(&info, "to", AR::from(end_heads))?;
+                c.call2(&JsValue::undefined(), &patches.into(), &info)?;
             }
         }
 
-        self.finalize_exposed(&object, exposed, &meta)?;
-
         Ok(object.into())
     }
 
@@ -616,10 +633,11 @@ impl Automerge {
         // committed.
         // If we pop the patches then we won't be able to revert them.
 
-        let patches = self.doc.observer().take_patches();
+        let heads = self.doc.get_heads();
+        let (patches, _heads) = self.doc.observer().take_patches(heads);
         let result = Array::new();
         for p in patches {
-            result.push(&p.try_into()?);
+            result.push(&interop::JsPatch(p).try_into()?);
         }
         Ok(result)
     }
@@ -702,17 +720,12 @@ impl Automerge {
     #[wasm_bindgen(js_name = getHeads)]
     pub fn get_heads(&mut self) -> Array {
         let heads = self.doc.get_heads();
-        let heads: Array = heads
-            .iter()
-            .map(|h| JsValue::from_str(&hex::encode(h.0)))
-            .collect();
-        heads
+        AR::from(heads).into()
     }
 
     #[wasm_bindgen(js_name = getActorId)]
     pub fn get_actor_id(&self) -> String {
-        let actor = self.doc.get_actor();
-        actor.to_string()
+        self.doc.get_actor().to_string()
     }
 
     #[wasm_bindgen(js_name = getLastLocalChange)]
@@ -775,7 +788,8 @@ impl Automerge {
     ) -> Result<JsValue, error::Materialize> {
         let (obj, obj_type) = self.import(obj).unwrap_or((ROOT, am::ObjType::Map));
         let heads = get_heads(heads)?;
-        let _patches = self.doc.observer().take_patches(); // throw away patches
+        let current_heads = self.doc.get_heads();
+        let _patches = self.doc.observer().take_patches(current_heads); // throw away patches
         Ok(self.export_object(&obj, obj_type.into(), heads.as_ref(), &meta)?)
     }
 
@@ -786,6 +800,75 @@ impl Automerge {
         let hash = self.doc.empty_change(options);
         JsValue::from_str(&hex::encode(hash))
     }
+
+    pub fn mark(
+        &mut self,
+        obj: JsValue,
+        range: JsValue,
+        name: JsValue,
+        value: JsValue,
+        datatype: JsValue,
+    ) -> Result<(), JsValue> {
+        let (obj, _) = self.import(obj)?;
+        let re = Regex::new(r"([\[\(])(\d+)\.\.(\d+)([\)\]])").unwrap();
+        let range = range.as_string().ok_or("range must be a string")?;
+        let cap = re.captures_iter(&range).next().ok_or(format!("(range={}) range must be in the form of (start..end] or [start..end) etc... () for sticky, [] for normal",range))?;
+        let start: usize = cap[2].parse().map_err(|_| to_js_err("invalid start"))?;
+        let end: usize = cap[3].parse().map_err(|_| to_js_err("invalid end"))?;
+        let left_sticky = &cap[1] == "(";
+        let right_sticky = &cap[4] == ")";
+        let name = name
+            .as_string()
+            .ok_or("invalid mark name")
+            .map_err(to_js_err)?;
+        let value = self
+            .import_scalar(&value, &datatype.as_string())
+            .ok_or_else(|| to_js_err("invalid value"))?;
+        self.doc
+            .mark(
+                &obj,
+                am::marks::Mark::new(name, value, start, end),
+                am::marks::ExpandMark::from(left_sticky, right_sticky),
+            )
+            .map_err(to_js_err)?;
+        Ok(())
+    }
+
+    pub fn unmark(
+        &mut self,
+        obj: JsValue,
+        key: JsValue,
+        start: f64,
+        end: f64,
+    ) -> Result<(), JsValue> {
+        let (obj, _) = self.import(obj)?;
+        let key = key.as_string().ok_or("key must be a string")?;
+        self.doc
+            .unmark(&obj, &key, start as usize, end as usize)
+            .map_err(to_js_err)?;
+        Ok(())
+    }
+
+    pub fn marks(&mut self, obj: JsValue, heads: Option<Array>) -> Result<JsValue, JsValue> {
+        let (obj, _) = self.import(obj)?;
+        let heads = get_heads(heads)?;
+        let marks = if let Some(heads) = heads {
+            self.doc.marks_at(obj, &heads).map_err(to_js_err)?
+        } else {
+            self.doc.marks(obj).map_err(to_js_err)?
+        };
+        let result = Array::new();
+        for m in marks {
+            let mark = Object::new();
+            let (_datatype, value) = alloc(&m.value().clone().into(), self.text_rep);
+            js_set(&mark, "name", m.name())?;
+            js_set(&mark, "value", value)?;
+            js_set(&mark, "start", m.start as i32)?;
+            js_set(&mark, "end", m.end as i32)?;
+            result.push(&mark.into());
+        }
+        Ok(result.into())
+    }
 }
 
 #[wasm_bindgen(js_name = create)]
@@ -812,7 +895,7 @@ pub fn load(
         TextRepresentation::Array
     };
     let mut doc = am::AutoCommitWithObs::<UnObserved>::load(&data)?
-        .with_observer(Observer::default().with_text_rep(text_rep))
+        .with_observer(ToggleObserver::default().with_text_rep(text_rep.into()))
         .with_encoding(TextEncoding::Utf16);
     if let Some(s) = actor {
         let actor =
diff --git a/rust/automerge-wasm/src/observer.rs b/rust/automerge-wasm/src/observer.rs
deleted file mode 100644
index 2351c762..00000000
--- a/rust/automerge-wasm/src/observer.rs
+++ /dev/null
@@ -1,518 +0,0 @@
-#![allow(dead_code)]
-
-use std::borrow::Cow;
-
-use crate::{
-    interop::{self, alloc, js_set},
-    TextRepresentation,
-};
-use automerge::{ObjId, OpObserver, Prop, ReadDoc, ScalarValue, Value};
-use js_sys::{Array, Object};
-use wasm_bindgen::prelude::*;
-
-use crate::sequence_tree::SequenceTree;
-
-#[derive(Debug, Clone, Default)]
-pub(crate) struct Observer {
-    enabled: bool,
-    patches: Vec<Patch>,
-    text_rep: TextRepresentation,
-}
-
-impl Observer {
-    pub(crate) fn take_patches(&mut self) -> Vec<Patch> {
-        std::mem::take(&mut self.patches)
-    }
-    pub(crate) fn enable(&mut self, enable: bool) -> bool {
-        if self.enabled && !enable {
-            self.patches.truncate(0)
-        }
-        let old_enabled = self.enabled;
-        self.enabled = enable;
-        old_enabled
-    }
-
-    fn get_path<R: ReadDoc>(&mut self, doc: &R, obj: &ObjId) -> Option<Vec<(ObjId, Prop)>> {
-        match doc.parents(obj) {
-            Ok(parents) => parents.visible_path(),
-            Err(e) => {
-                automerge::log!("error generating patch : {:?}", e);
-                None
-            }
-        }
-    }
-
-    pub(crate) fn with_text_rep(mut self, text_rep: TextRepresentation) -> Self {
-        self.text_rep = text_rep;
-        self
-    }
-
-    pub(crate) fn set_text_rep(&mut self, text_rep: TextRepresentation) {
-        self.text_rep = text_rep;
-    }
-}
-
-#[derive(Debug, Clone)]
-pub(crate) enum Patch {
-    PutMap {
-        obj: ObjId,
-        path: Vec<(ObjId, Prop)>,
-        key: String,
-        value: (Value<'static>, ObjId),
-        expose: bool,
-    },
-    PutSeq {
-        obj: ObjId,
-        path: Vec<(ObjId, Prop)>,
-        index: usize,
-        value: (Value<'static>, ObjId),
-        expose: bool,
-    },
-    Insert {
-        obj: ObjId,
-        path: Vec<(ObjId, Prop)>,
-        index: usize,
-        values: SequenceTree<(Value<'static>, ObjId)>,
-    },
-    SpliceText {
-        obj: ObjId,
-        path: Vec<(ObjId, Prop)>,
-        index: usize,
-        value: SequenceTree<u16>,
-    },
-    Increment {
-        obj: ObjId,
-        path: Vec<(ObjId, Prop)>,
-        prop: Prop,
-        value: i64,
-    },
-    DeleteMap {
-        obj: ObjId,
-        path: Vec<(ObjId, Prop)>,
-        key: String,
-    },
-    DeleteSeq {
-        obj: ObjId,
-        path: Vec<(ObjId, Prop)>,
-        index: usize,
-        length: usize,
-    },
-}
-
-impl OpObserver for Observer {
-    fn insert<R: ReadDoc>(
-        &mut self,
-        doc: &R,
-        obj: ObjId,
-        index: usize,
-        tagged_value: (Value<'_>, ObjId),
-    ) {
-        if self.enabled {
-            let value = (tagged_value.0.to_owned(), tagged_value.1);
-            if let Some(Patch::Insert {
-                obj: tail_obj,
-                index: tail_index,
-                values,
-                ..
-            }) = self.patches.last_mut()
-            {
-                let range = *tail_index..=*tail_index + values.len();
-                if tail_obj == &obj && range.contains(&index) {
-                    values.insert(index - *tail_index, value);
-                    return;
-                }
-            }
-            if let Some(path) = self.get_path(doc, &obj) {
-                let mut values = SequenceTree::new();
-                values.push(value);
-                let patch = Patch::Insert {
-                    path,
-                    obj,
-                    index,
-                    values,
-                };
-                self.patches.push(patch);
-            }
-        }
-    }
-
-    fn splice_text<R: ReadDoc>(&mut self, doc: &R, obj: ObjId, index: usize, value: &str) {
-        if self.enabled {
-            if self.text_rep == TextRepresentation::Array {
-                for (i, c) in value.chars().enumerate() {
-                    self.insert(
-                        doc,
-                        obj.clone(),
-                        index + i,
-                        (
-                            Value::Scalar(Cow::Owned(ScalarValue::Str(c.to_string().into()))),
-                            ObjId::Root, // We hope this is okay
-                        ),
-                    );
-                }
-                return;
-            }
-            if let Some(Patch::SpliceText {
-                obj: tail_obj,
-                index: tail_index,
-                value: prev_value,
-                ..
-            }) = self.patches.last_mut()
-            {
-                let range = *tail_index..=*tail_index + prev_value.len();
-                if tail_obj == &obj && range.contains(&index) {
-                    let i = index - *tail_index;
-                    for (n, ch) in value.encode_utf16().enumerate() {
-                        prev_value.insert(i + n, ch)
-                    }
-                    return;
-                }
-            }
-            if let Some(path) = self.get_path(doc, &obj) {
-                let mut v = SequenceTree::new();
-                for ch in value.encode_utf16() {
-                    v.push(ch)
-                }
-                let patch = Patch::SpliceText {
-                    path,
-                    obj,
-                    index,
-                    value: v,
-                };
-                self.patches.push(patch);
-            }
-        }
-    }
-
-    fn delete_seq<R: ReadDoc>(&mut self, doc: &R, obj: ObjId, index: usize, length: usize) {
-        if self.enabled {
-            match self.patches.last_mut() {
-                Some(Patch::SpliceText {
-                    obj: tail_obj,
-                    index: tail_index,
-                    value,
-                    ..
-                }) => {
-                    let range = *tail_index..*tail_index + value.len();
-                    if tail_obj == &obj
-                        && range.contains(&index)
-                        && range.contains(&(index + length - 1))
-                    {
-                        for _ in 0..length {
-                            value.remove(index - *tail_index);
-                        }
-                        return;
-                    }
-                }
-                Some(Patch::Insert {
-                    obj: tail_obj,
-                    index: tail_index,
-                    values,
-                    ..
-                }) => {
-                    let range = *tail_index..*tail_index + values.len();
-                    if tail_obj == &obj
-                        && range.contains(&index)
-                        && range.contains(&(index + length - 1))
-                    {
-                        for _ in 0..length {
-                            values.remove(index - *tail_index);
-                        }
-                        return;
-                    }
-                }
-                Some(Patch::DeleteSeq {
-                    obj: tail_obj,
-                    index: tail_index,
-                    length: tail_length,
-                    ..
-                }) => {
-                    if tail_obj == &obj && index == *tail_index {
-                        *tail_length += length;
-                        return;
-                    }
-                }
-                _ => {}
-            }
-            if let Some(path) = self.get_path(doc, &obj) {
-                let patch = Patch::DeleteSeq {
-                    path,
-                    obj,
-                    index,
-                    length,
-                };
-                self.patches.push(patch)
-            }
-        }
-    }
-
-    fn delete_map<R: ReadDoc>(&mut self, doc: &R, obj: ObjId, key: &str) {
-        if self.enabled {
-            if let Some(path) = self.get_path(doc, &obj) {
-                let patch = Patch::DeleteMap {
-                    path,
-                    obj,
-                    key: key.to_owned(),
-                };
-                self.patches.push(patch)
-            }
-        }
-    }
-
-    fn put<R: ReadDoc>(
-        &mut self,
-        doc: &R,
-        obj: ObjId,
-        prop: Prop,
-        tagged_value: (Value<'_>, ObjId),
-        _conflict: bool,
-    ) {
-        if self.enabled {
-            let expose = false;
-            if let Some(path) = self.get_path(doc, &obj) {
-                let value = (tagged_value.0.to_owned(), tagged_value.1);
-                let patch = match prop {
-                    Prop::Map(key) => Patch::PutMap {
-                        path,
-                        obj,
-                        key,
-                        value,
-                        expose,
-                    },
-                    Prop::Seq(index) => Patch::PutSeq {
-                        path,
-                        obj,
-                        index,
-                        value,
-                        expose,
-                    },
-                };
-                self.patches.push(patch);
-            }
-        }
-    }
-
-    fn expose<R: ReadDoc>(
-        &mut self,
-        doc: &R,
-        obj: ObjId,
-        prop: Prop,
-        tagged_value: (Value<'_>, ObjId),
-        _conflict: bool,
-    ) {
-        if self.enabled {
-            let expose = true;
-            if let Some(path) = self.get_path(doc, &obj) {
-                let value = (tagged_value.0.to_owned(), tagged_value.1);
-                let patch = match prop {
-                    Prop::Map(key) => Patch::PutMap {
-                        path,
-                        obj,
-                        key,
-                        value,
-                        expose,
-                    },
-                    Prop::Seq(index) => Patch::PutSeq {
-                        path,
-                        obj,
-                        index,
-                        value,
-                        expose,
-                    },
-                };
-                self.patches.push(patch);
-            }
-        }
-    }
-
-    fn increment<R: ReadDoc>(
-        &mut self,
-        doc: &R,
-        obj: ObjId,
-        prop: Prop,
-        tagged_value: (i64, ObjId),
-    ) {
-        if self.enabled {
-            if let Some(path) = self.get_path(doc, &obj) {
-                let value = tagged_value.0;
-                self.patches.push(Patch::Increment {
-                    path,
-                    obj,
-                    prop,
-                    value,
-                })
-            }
-        }
-    }
-
-    fn text_as_seq(&self) -> bool {
-        self.text_rep == TextRepresentation::Array
-    }
-}
-
-impl automerge::op_observer::BranchableObserver for Observer {
-    fn merge(&mut self, other: &Self) {
-        self.patches.extend_from_slice(other.patches.as_slice())
-    }
-
-    fn branch(&self) -> Self {
-        Observer {
-            patches: vec![],
-            enabled: self.enabled,
-            text_rep: self.text_rep,
-        }
-    }
-}
-
-fn prop_to_js(p: &Prop) -> JsValue {
-    match p {
-        Prop::Map(key) => JsValue::from_str(key),
-        Prop::Seq(index) => JsValue::from_f64(*index as f64),
-    }
-}
-
-fn export_path(path: &[(ObjId, Prop)], end: &Prop) -> Array {
-    let result = Array::new();
-    for p in path {
-        result.push(&prop_to_js(&p.1));
-    }
-    result.push(&prop_to_js(end));
-    result
-}
-
-impl Patch {
-    pub(crate) fn path(&self) -> &[(ObjId, Prop)] {
-        match &self {
-            Self::PutMap { path, .. } => path.as_slice(),
-            Self::PutSeq { path, .. } => path.as_slice(),
-            Self::Increment { path, .. } => path.as_slice(),
-            Self::Insert { path, .. } => path.as_slice(),
-            Self::SpliceText { path, .. } => path.as_slice(),
-            Self::DeleteMap { path, .. } => path.as_slice(),
-            Self::DeleteSeq { path, .. } => path.as_slice(),
-        }
-    }
-
-    pub(crate) fn obj(&self) -> &ObjId {
-        match &self {
-            Self::PutMap { obj, .. } => obj,
-            Self::PutSeq { obj, .. } => obj,
-            Self::Increment { obj, .. } => obj,
-            Self::Insert { obj, .. } => obj,
-            Self::SpliceText { obj, .. } => obj,
-            Self::DeleteMap { obj, .. } => obj,
-            Self::DeleteSeq { obj, .. } => obj,
-        }
-    }
-}
-
-impl TryFrom<Patch> for JsValue {
-    type Error = interop::error::Export;
-
-    fn try_from(p: Patch) -> Result<Self, Self::Error> {
-        let result = Object::new();
-        match p {
-            Patch::PutMap {
-                path, key, value, ..
-            } => {
-                js_set(&result, "action", "put")?;
-                js_set(
-                    &result,
-                    "path",
-                    export_path(path.as_slice(), &Prop::Map(key)),
-                )?;
-                js_set(
-                    &result,
-                    "value",
-                    alloc(&value.0, TextRepresentation::String).1,
-                )?;
-                Ok(result.into())
-            }
-            Patch::PutSeq {
-                path, index, value, ..
-            } => {
-                js_set(&result, "action", "put")?;
-                js_set(
-                    &result,
-                    "path",
-                    export_path(path.as_slice(), &Prop::Seq(index)),
-                )?;
-                js_set(
-                    &result,
-                    "value",
-                    alloc(&value.0, TextRepresentation::String).1,
-                )?;
-                Ok(result.into())
-            }
-            Patch::Insert {
-                path,
-                index,
-                values,
-                ..
-            } => {
-                js_set(&result, "action", "insert")?;
-                js_set(
-                    &result,
-                    "path",
-                    export_path(path.as_slice(), &Prop::Seq(index)),
-                )?;
-                js_set(
-                    &result,
-                    "values",
-                    values
-                        .iter()
-                        .map(|v| alloc(&v.0, TextRepresentation::String).1)
-                        .collect::<Array>(),
-                )?;
-                Ok(result.into())
-            }
-            Patch::SpliceText {
-                path, index, value, ..
-            } => {
-                js_set(&result, "action", "splice")?;
-                js_set(
-                    &result,
-                    "path",
-                    export_path(path.as_slice(), &Prop::Seq(index)),
-                )?;
-                let bytes: Vec<u16> = value.iter().cloned().collect();
-                js_set(&result, "value", String::from_utf16_lossy(bytes.as_slice()))?;
-                Ok(result.into())
-            }
-            Patch::Increment {
-                path, prop, value, ..
-            } => {
-                js_set(&result, "action", "inc")?;
-                js_set(&result, "path", export_path(path.as_slice(), &prop))?;
-                js_set(&result, "value", &JsValue::from_f64(value as f64))?;
-                Ok(result.into())
-            }
-            Patch::DeleteMap { path, key, .. } => {
-                js_set(&result, "action", "del")?;
-                js_set(
-                    &result,
-                    "path",
-                    export_path(path.as_slice(), &Prop::Map(key)),
-                )?;
-                Ok(result.into())
-            }
-            Patch::DeleteSeq {
-                path,
-                index,
-                length,
-                ..
-            } => {
-                js_set(&result, "action", "del")?;
-                js_set(
-                    &result,
-                    "path",
-                    export_path(path.as_slice(), &Prop::Seq(index)),
-                )?;
-                if length > 1 {
-                    js_set(&result, "length", length)?;
-                }
-                Ok(result.into())
-            }
-        }
-    }
-}
diff --git a/rust/automerge-wasm/test/marks.ts b/rust/automerge-wasm/test/marks.ts
new file mode 100644
index 00000000..0a2d4433
--- /dev/null
+++ b/rust/automerge-wasm/test/marks.ts
@@ -0,0 +1,572 @@
+import { describe, it } from 'mocha';
+//@ts-ignore
+import assert from 'assert'
+//@ts-ignore
+import { create, load, Automerge, encodeChange, decodeChange } from '..'
+import { v4 as uuid } from "uuid"
+
+
+let util = require('util')
+
+describe('Automerge', () => {
+  describe('marks', () => {
+    it('should handle marks [..]', () => {
+      let doc = create(true)
+      let list = doc.putObject("_root", "list", "")
+      doc.splice(list, 0, 0, "aaabbbccc")
+      doc.mark(list, "[3..6]", "bold" , true)
+      let text = doc.text(list)
+      let marks = doc.marks(list);
+      assert.deepStrictEqual(marks, [{ name: 'bold', value: true, start: 3, end: 6 }])
+      doc.insert(list, 6, "A")
+      doc.insert(list, 3, "A")
+      marks = doc.marks(list);
+      assert.deepStrictEqual(marks, [{ name: 'bold', value: true, start: 4, end: 7 }])
+    })
+
+    it('should handle mark and unmark', () => {
+      let doc = create(true)
+      let list = doc.putObject("_root", "list", "")
+      doc.splice(list, 0, 0, "aaabbbccc")
+      doc.mark(list, "[2..8]", "bold" , true)
+      let marks = doc.marks(list);
+      assert.deepStrictEqual(marks, [{ name: 'bold', value: true, start: 2, end: 8 }])
+      doc.unmark(list, 'bold', 4, 6)
+      doc.insert(list, 7, "A")
+      doc.insert(list, 3, "A")
+      marks = doc.marks(list);
+      assert.deepStrictEqual(marks, [
+        { name: 'bold', value: true, start: 2, end: 5 },
+        { name: 'bold', value: true, start: 7, end: 10 },
+      ])
+    })
+
+    it('should handle mark and unmark of overlapping marks', () => {
+      let doc = create(true)
+      let list = doc.putObject("_root", "list", "")
+      doc.splice(list, 0, 0, "aaabbbccc")
+      doc.mark(list, "[2..6]", "bold" , true)
+      doc.mark(list, "[5..8]", "bold" , true)
+      doc.mark(list, "[3..6]", "underline" , true)
+      let marks = doc.marks(list);
+      assert.deepStrictEqual(marks, [
+        { name: 'underline', value: true, start: 3, end: 6 },
+        { name: 'bold', value: true, start: 2, end: 8 },
+      ])
+      doc.unmark(list, 'bold', 4, 6)
+      doc.insert(list, 7, "A")
+      doc.insert(list, 3, "A")
+      marks = doc.marks(list);
+      assert.deepStrictEqual(marks, [
+        { name: 'bold', value: true, start: 2, end: 5 },
+        { name: 'underline', value: true, start: 4, end: 7 },
+        { name: 'bold', value: true, start: 7, end: 10 },
+      ])
+      doc.unmark(list, 'bold', 0, 11)
+      marks = doc.marks(list);
+      assert.deepStrictEqual(marks, [
+        { name: 'underline', value: true, start: 4, end: 7 }
+      ])
+    })
+
+    it('should handle marks [..] at the beginning of a string', () => {
+      let doc = create(true)
+      let list = doc.putObject("_root", "list", "")
+      doc.splice(list, 0, 0, "aaabbbccc")
+      doc.mark(list, "[0..3]", "bold", true)
+      let marks = doc.marks(list);
+      assert.deepStrictEqual(marks, [{ name: 'bold', value: true, start: 0, end: 3 }])
+
+      let doc2 = doc.fork()
+      doc2.insert(list, 0, "A")
+      doc2.insert(list, 4, "B")
+      doc.merge(doc2)
+      marks = doc.marks(list);
+      assert.deepStrictEqual(marks, [{ name: 'bold', value: true, start: 1, end: 4 }])
+    })
+
+    it('should handle marks [..] with splice', () => {
+      let doc = create(true)
+      let list = doc.putObject("_root", "list", "")
+      doc.splice(list, 0, 0, "aaabbbccc")
+      doc.mark(list, "[0..3]", "bold", true)
+      let marks = doc.marks(list);
+      assert.deepStrictEqual(marks, [{ name: 'bold', value: true, start: 0, end: 3 }])
+
+      let doc2 = doc.fork()
+      doc2.splice(list, 0, 2, "AAA")
+      doc2.splice(list, 4, 0, "BBB")
+      doc.merge(doc2)
+      marks = doc.marks(list);
+      assert.deepStrictEqual(marks, [{ name: 'bold', value: true, start: 3, end: 4 }])
+    })
+
+    it('should handle marks across multiple forks', () => {
+      let doc = create(true)
+      let list = doc.putObject("_root", "list", "")
+      doc.splice(list, 0, 0, "aaabbbccc")
+      doc.mark(list, "[0..3]", "bold", true)
+      let marks = doc.marks(list);
+      assert.deepStrictEqual(marks, [{ name: 'bold', value: true, start: 0, end: 3 }])
+
+      let doc2 = doc.fork()
+      doc2.splice(list, 1, 1, "Z") // replace 'aaa' with 'aZa' inside mark.
+
+      let doc3 = doc.fork()
+      doc3.insert(list, 0, "AAA") // should not be included in mark.
+
+      doc.merge(doc2)
+      doc.merge(doc3)
+
+      marks = doc.marks(list);
+      assert.deepStrictEqual(marks, [{ name: 'bold', value: true, start: 3, end: 6 }])
+    })
+
+    it('should handle marks with deleted ends [..]', () => {
+      let doc = create(true)
+      let list = doc.putObject("_root", "list", "")
+
+      doc.splice(list, 0, 0, "aaabbbccc")
+      doc.mark(list, "[3..6]", "bold" , true)
+      let marks = doc.marks(list);
+      assert.deepStrictEqual(marks, [{ name: 'bold', value: true, start: 3, end: 6 }])
+      doc.delete(list,5);
+      doc.delete(list,5);
+      doc.delete(list,2);
+      doc.delete(list,2);
+      marks = doc.marks(list);
+      assert.deepStrictEqual(marks, [{ name: 'bold', value: true, start: 2, end: 3 }])
+      doc.insert(list, 3, "A")
+      doc.insert(list, 2, "A")
+      marks = doc.marks(list);
+      assert.deepStrictEqual(marks, [{ name: 'bold', value: true, start: 3, end: 4 }])
+    })
+
+    it('should handle sticky marks (..)', () => {
+      let doc = create(true)
+      let list = doc.putObject("_root", "list", "")
+      doc.splice(list, 0, 0, "aaabbbccc")
+      doc.mark(list, "(3..6)", "bold" , true)
+      let marks = doc.marks(list);
+      assert.deepStrictEqual(marks, [{ name: 'bold', value: true, start: 3, end: 6 }])
+      doc.insert(list, 6, "A")
+      doc.insert(list, 3, "A")
+      marks = doc.marks(list);
+      assert.deepStrictEqual(marks, [{ name: 'bold', value: true, start: 3, end: 8 }])
+    })
+
+    it('should handle sticky marks with deleted ends (..)', () => {
+      let doc = create(true)
+      let list = doc.putObject("_root", "list", "")
+      doc.splice(list, 0, 0, "aaabbbccc")
+      doc.mark(list, "(3..6)", "bold" , true)
+      let marks = doc.marks(list);
+      assert.deepStrictEqual(marks, [{ name: 'bold', value: true, start: 3, end: 6 }])
+      doc.delete(list,5);
+      doc.delete(list,5);
+      doc.delete(list,2);
+      doc.delete(list,2);
+      marks = doc.marks(list);
+      assert.deepStrictEqual(marks, [{ name: 'bold', value: true, start: 2, end: 3 }])
+      doc.insert(list, 3, "A")
+      doc.insert(list, 2, "A")
+      marks = doc.marks(list);
+      assert.deepStrictEqual(marks, [{ name: 'bold', value: true, start: 2, end: 5 }])
+
+      // make sure save/load can handle marks
+
+      let saved = doc.save()
+      let doc2 = load(saved,true)
+      marks = doc2.marks(list);
+      assert.deepStrictEqual(marks, [{ name: 'bold', value: true, start: 2, end: 5 }])
+
+      assert.deepStrictEqual(doc.getHeads(), doc2.getHeads())
+      assert.deepStrictEqual(doc.save(), doc2.save())
+    })
+
+    it('should handle overlapping marks', () => {
+      let doc : Automerge = create(true, "aabbcc")
+      let list = doc.putObject("_root", "list", "")
+      doc.splice(list, 0, 0, "the quick fox jumps over the lazy dog")
+      doc.mark(list, "[0..37]", "bold" , true)
+      doc.mark(list, "[4..19]", "itallic" , true)
+      let id = uuid(); // we want each comment to be unique so give it a unique id
+      doc.mark(list, "[10..13]", `comment:${id}` , "foxes are my favorite animal!")
+      doc.commit("marks");
+      let marks = doc.marks(list);
+      assert.deepStrictEqual(marks, [
+        { name: `comment:${id}`, start: 10, end: 13,  value: 'foxes are my favorite animal!' },
+        { name: 'itallic', start: 4, end: 19, value: true },
+        { name: 'bold', start: 0, end: 37, value: true }
+      ])
+      let text = doc.text(list);
+      assert.deepStrictEqual(text, "the quick fox jumps over the lazy dog");
+
+      let all = doc.getChanges([])
+      let decoded = all.map((c) => decodeChange(c))
+      let util = require('util');
+      let encoded = decoded.map((c) => encodeChange(c))
+      let decoded2 = encoded.map((c) => decodeChange(c))
+      let doc2 = create(true);
+      doc2.applyChanges(encoded)
+
+      assert.deepStrictEqual(doc.marks(list) , doc2.marks(list))
+      assert.deepStrictEqual(doc.save(), doc2.save())
+    })
+
+    it('generates patches for marks made locally', () => {
+      let doc : Automerge = create(true, "aabbcc")
+      doc.enablePatches(true)
+      let list = doc.putObject("_root", "list", "")
+      doc.splice(list, 0, 0, "the quick fox jumps over the lazy dog")
+      let h1 = doc.getHeads()
+      doc.mark(list, "[0..37]", "bold" , true)
+      doc.mark(list, "[4..19]", "itallic" , true)
+      let id = uuid(); // we want each comment to be unique so give it a unique id
+      doc.mark(list, "[10..13]", `comment:${id}` , "foxes are my favorite animal!")
+      doc.commit("marks");
+      let h2 = doc.getHeads()
+      let patches = doc.popPatches();
+      let util = require('util')
+      assert.deepEqual(patches, [
+        { action: 'put', path: [ 'list' ], value: '' },
+        {
+          action: 'splice', path: [ 'list', 0 ],
+          value: 'the quick fox jumps over the lazy dog'
+        },
+        {
+          action: 'mark', path: [ 'list' ],
+          marks: [
+            { name: 'bold', value: true, start: 0, end: 37  },
+            { name: 'itallic', value: true, start: 4, end: 19 },
+            { name: `comment:${id}`, value: 'foxes are my favorite animal!', start: 10, end: 13 }
+          ]
+        }
+      ]);
+    })
+
+    it('marks should create patches that respect marks that supersede it', () => {
+
+      let doc1 : Automerge = create(true, "aabbcc")
+      let list = doc1.putObject("_root", "list", "")
+      doc1.splice(list, 0, 0, "the quick fox jumps over the lazy dog")
+
+      let doc2 = load(doc1.save(),true);
+
+      let doc3 = load(doc1.save(),true);
+      doc3.enablePatches(true)
+
+      doc1.put("/","foo", "bar"); // make a change to our op counter is higher than doc2
+      doc1.mark(list, "[0..5]", "x", "a")
+      doc1.mark(list, "[8..11]", "x", "b")
+
+      doc2.mark(list, "[4..13]", "x", "c");
+
+      doc3.merge(doc1)
+      doc3.merge(doc2)
+
+      let patches = doc3.popPatches();
+
+      assert.deepEqual(patches, [
+          { action: 'put', path: [ 'foo' ], value: 'bar' },
+          {
+            action: 'mark',
+            path: [ 'list' ],
+            marks: [
+              { name: 'x', value: 'a', start: 0, end: 5 },
+              { name: 'x', value: 'b', start: 8, end: 11 },
+              { name: 'x', value: 'c', start: 5, end: 8 },
+              { name: 'x', value: 'c', start: 11, end: 13 },
+            ]
+          },
+        ]);
+    })
+  })
+  describe('loading marks', () => {
+    it('a mark will appear on load', () => {
+      let doc1 : Automerge = create(true, "aabbcc")
+      doc1.enablePatches(true)
+
+      let list = doc1.putObject("_root", "list", "")
+      doc1.splice(list, 0, 0, "the quick fox jumps over the lazy dog")
+      doc1.mark(list, "[5..10]", "xxx", "aaa")
+
+      let patches1 = doc1.popPatches().filter((p:any) => p.action == "mark")
+
+      assert.deepEqual(patches1, [{
+        action: 'mark', path: [ 'list' ], marks: [ { name: 'xxx', value: 'aaa', start: 5, end: 10 }],
+      }]);
+
+      let doc2 : Automerge = create(true);
+      doc2.enablePatches(true)
+      doc2.loadIncremental(doc1.save())
+
+      let patches2 = doc2.popPatches().filter((p:any) => p.action == "mark")
+
+      assert.deepEqual(patches2, [{
+        action: 'mark', path: ['list'], marks: [ { name: 'xxx', value: 'aaa', start: 5, end: 10}],
+      }]);
+    })
+
+    it('a overlapping marks will coalesse on load', () => {
+      let doc1 : Automerge = create(true, "aabbcc")
+      doc1.enablePatches(true)
+
+      let list = doc1.putObject("_root", "list", "")
+      doc1.splice(list, 0, 0, "the quick fox jumps over the lazy dog")
+      doc1.mark(list, "[5..15]", "xxx", "aaa")
+      doc1.mark(list, "[10..20]", "xxx", "aaa")
+      doc1.mark(list, "[15..25]", "xxx", "aaa")
+
+      let patches1 = doc1.popPatches().filter((p:any) => p.action == "mark")
+
+      assert.deepEqual(patches1, [
+        { action: 'mark', path: [ 'list' ], marks: [
+          { name: 'xxx', value: 'aaa', start: 5, end: 15 },
+          { name: 'xxx', value: 'aaa', start: 10, end: 20 },
+          { name: 'xxx', value: 'aaa', start: 15, end: 25 },
+        ] },
+      ]);
+
+      let doc2 : Automerge = create(true);
+      doc2.enablePatches(true)
+      doc2.loadIncremental(doc1.save())
+
+      let patches2 = doc2.popPatches().filter((p:any) => p.action == "mark")
+
+      assert.deepEqual(patches2, [
+        { action: 'mark', path: ['list'], marks: [ { name: 'xxx', value: 'aaa', start: 5, end: 25}] },
+      ]);
+    })
+
+    it('coalesse handles different values', () => {
+      let doc1 : Automerge = create(true, "aabbcc")
+      doc1.enablePatches(true)
+
+      let list = doc1.putObject("_root", "list", "")
+      doc1.splice(list, 0, 0, "the quick fox jumps over the lazy dog")
+      doc1.mark(list, "[5..15]", "xxx", "aaa")
+      doc1.mark(list, "[10..20]", "xxx", "bbb")
+      doc1.mark(list, "[15..25]", "xxx", "aaa")
+
+      let patches1 = doc1.popPatches().filter((p:any) => p.action == "mark")
+
+      assert.deepEqual(patches1, [
+        { action: 'mark', path: [ 'list' ], marks: [
+          { name: 'xxx', value: 'aaa', start: 5, end: 15 },
+          { name: 'xxx', value: 'bbb', start: 10, end: 20 },
+          { name: 'xxx', value: 'aaa', start: 15, end: 25 },
+        ]}
+      ]);
+
+      let doc2 : Automerge = create(true);
+      doc2.enablePatches(true)
+      doc2.loadIncremental(doc1.save())
+
+      let patches2 = doc2.popPatches().filter((p:any) => p.action == "mark")
+
+      assert.deepEqual(patches2, [
+        { action: 'mark', path: ['list'], marks: [
+          { name: 'xxx', value: 'aaa', start: 5, end: 10 },
+          { name: 'xxx', value: 'bbb', start: 10, end: 15 },
+          { name: 'xxx', value: 'aaa', start: 15, end: 25 },
+        ]},
+      ]);
+    })
+
+    it('wont coalesse handles different names', () => {
+      let doc1 : Automerge = create(true, "aabbcc")
+      doc1.enablePatches(true)
+
+      let list = doc1.putObject("_root", "list", "")
+      doc1.splice(list, 0, 0, "the quick fox jumps over the lazy dog")
+      doc1.mark(list, "[5..15]", "xxx", "aaa")
+      doc1.mark(list, "[10..20]", "yyy", "aaa")
+      doc1.mark(list, "[15..25]", "zzz", "aaa")
+
+      let patches1 = doc1.popPatches().filter((p:any) => p.action == "mark")
+
+      assert.deepEqual(patches1, [
+        { action: 'mark', path: [ 'list' ], marks: [
+          { name: 'xxx', value: 'aaa', start: 5, end:15 },
+          { name: 'yyy', value: 'aaa', start: 10, end: 20 },
+          { name: 'zzz', value: 'aaa', start: 15, end: 25 },
+          ]}
+      ]);
+
+      let doc2 : Automerge = create(true);
+      doc2.enablePatches(true)
+      doc2.loadIncremental(doc1.save())
+
+      let patches2 = doc2.popPatches().filter((p:any) => p.action == "mark")
+
+      assert.deepEqual(patches2, [
+        { action: 'mark', path: [ 'list' ], marks: [
+          { name: 'xxx', value: 'aaa', start: 5, end: 15 },
+          { name: 'yyy', value: 'aaa', start: 10, end: 20 },
+          { name: 'zzz', value: 'aaa', start: 15, end: 25 },
+        ]}
+      ]);
+    })
+
+    it('coalesse handles async merge', () => {
+      let doc1 : Automerge = create(true, "aabbcc")
+      doc1.enablePatches(true)
+
+      let list = doc1.putObject("_root", "list", "")
+      doc1.splice(list, 0, 0, "the quick fox jumps over the lazy dog")
+
+      let doc2 = doc1.fork()
+
+      doc1.put("/", "key1", "value"); // incrementing op counter so we win vs doc2
+      doc1.put("/", "key2", "value"); // incrementing op counter so we win vs doc2
+      doc1.mark(list, "[10..20]", "xxx", "aaa")
+      doc1.mark(list, "[15..25]", "xxx", "aaa")
+
+      doc2.mark(list, "[5..30]" , "xxx", "bbb")
+
+      doc1.merge(doc2)
+
+      let patches1 = doc1.popPatches().filter((p:any) => p.action == "mark")
+
+      assert.deepEqual(patches1, [
+        { action: 'mark', path: [ 'list' ], marks: [
+            { name: 'xxx', value: 'aaa', start: 10, end: 20 },
+            { name: 'xxx', value: 'aaa', start: 15, end: 25 },
+            { name: 'xxx', value: 'bbb', start: 5, end: 10 },
+            { name: 'xxx', value: 'bbb', start: 25, end: 30 },
+          ]
+        },
+      ]);
+
+      let doc3 : Automerge = create(true);
+      doc3.enablePatches(true)
+      doc3.loadIncremental(doc1.save())
+
+      let patches2 = doc3.popPatches().filter((p:any) => p.action == "mark")
+
+      let marks = doc3.marks(list)
+
+      assert.deepEqual(marks, [
+          { name: 'xxx', value: 'bbb', start: 5, end: 10 },
+          { name: 'xxx', value: 'aaa', start: 10, end: 25 },
+          { name: 'xxx', value: 'bbb', start: 25, end: 30  },
+      ]);
+
+      assert.deepEqual(patches2, [{ action: 'mark', path: [ 'list' ], marks }]);
+    })
+
+    it('does not show marks hidden in merge', () => {
+      let doc1 : Automerge = create(true, "aabbcc")
+      doc1.enablePatches(true)
+
+      let list = doc1.putObject("_root", "list", "")
+      doc1.splice(list, 0, 0, "the quick fox jumps over the lazy dog")
+
+      let doc2 = doc1.fork()
+
+      doc1.put("/", "key1", "value"); // incrementing op counter so we win vs doc2
+      doc1.put("/", "key2", "value"); // incrementing op counter so we win vs doc2
+      doc1.mark(list, "[10..20]", "xxx", "aaa")
+      doc1.mark(list, "[15..25]", "xxx", "aaa")
+
+      doc2.mark(list, "[11..24]" , "xxx", "bbb")
+
+      doc1.merge(doc2)
+
+      let patches1 = doc1.popPatches().filter((p:any) => p.action == "mark")
+
+      assert.deepEqual(patches1, [
+        { action: 'mark', path: [ 'list' ], marks: [
+            { name: 'xxx', value: 'aaa', start: 10, end: 20 },
+            { name: 'xxx', value: 'aaa', start: 15, end: 25 },
+          ]
+        },
+      ]);
+
+      let doc3 : Automerge = create(true);
+      doc3.enablePatches(true)
+      doc3.loadIncremental(doc1.save())
+
+      let patches2 = doc3.popPatches().filter((p:any) => p.action == "mark")
+
+      assert.deepEqual(patches2, [
+        { action: 'mark', path: [ 'list' ], marks: [
+          { name: 'xxx', value: 'aaa', start: 10, end: 25 },
+        ]}
+      ]);
+    })
+
+    it('coalesse disconnected marks with async merge', () => {
+      let doc1 : Automerge = create(true, "aabbcc")
+      doc1.enablePatches(true)
+
+      let list = doc1.putObject("_root", "list", "")
+      doc1.splice(list, 0, 0, "the quick fox jumps over the lazy dog")
+
+      let doc2 = doc1.fork()
+
+      doc1.put("/", "key1", "value"); // incrementing op counter so we win vs doc2
+      doc1.put("/", "key2", "value"); // incrementing op counter so we win vs doc2
+      doc1.mark(list, "[5..11]", "xxx", "aaa")
+      doc1.mark(list, "[19..25]", "xxx", "aaa")
+
+      doc2.mark(list, "[10..20]" , "xxx", "aaa")
+
+      doc1.merge(doc2)
+
+      let patches1 = doc1.popPatches().filter((p:any) => p.action == "mark")
+
+      assert.deepEqual(patches1, [
+        { action: 'mark', path: [ 'list' ], marks: [
+            { name: 'xxx', value: 'aaa', start: 5, end: 11 },
+            { name: 'xxx', value: 'aaa', start: 19, end: 25 },
+            { name: 'xxx', value: 'aaa', start: 11, end: 19 },
+          ]
+        },
+      ]);
+
+      let doc3 : Automerge = create(true);
+      doc3.enablePatches(true)
+      doc3.loadIncremental(doc1.save())
+
+      let patches2 = doc3.popPatches().filter((p:any) => p.action == "mark")
+
+      assert.deepEqual(patches2, [
+        { action: 'mark', path: [ 'list' ], marks: [
+          { name: 'xxx', value: 'aaa', start: 5, end: 25 },
+        ]}
+      ]);
+    })
+    it('can get marks at a given heads', () => {
+      let doc1 : Automerge = create(true, "aabbcc")
+      doc1.enablePatches(true)
+
+      let list = doc1.putObject("_root", "list", "")
+      doc1.splice(list, 0, 0, "the quick fox jumps over the lazy dog")
+
+      let heads1 = doc1.getHeads();
+      let marks1 = doc1.marks(list);
+
+      doc1.mark(list, "[3..25]", "xxx", "aaa")
+
+      let heads2 = doc1.getHeads();
+      let marks2 = doc1.marks(list);
+
+      doc1.mark(list, "[4..11]", "yyy", "bbb")
+
+      let heads3 = doc1.getHeads();
+      let marks3 = doc1.marks(list);
+
+      doc1.unmark(list, "xxx", 9, 20)
+
+      let heads4 = doc1.getHeads();
+      let marks4 = doc1.marks(list);
+
+      assert.deepEqual(marks1, doc1.marks(list,heads1))
+      assert.deepEqual(marks2, doc1.marks(list,heads2))
+      assert.deepEqual(marks3, doc1.marks(list,heads3))
+      assert.deepEqual(marks4, doc1.marks(list,heads4))
+    })
+  })
+})
diff --git a/rust/automerge-wasm/test/test.ts b/rust/automerge-wasm/test/test.ts
index bb4f71e3..3e57d92c 100644
--- a/rust/automerge-wasm/test/test.ts
+++ b/rust/automerge-wasm/test/test.ts
@@ -1941,6 +1941,40 @@ describe('Automerge', () => {
       assert.deepEqual(mat.text, "ab011ij")
     })
 
+    it('propogates exceptions thrown in patch callback', () => {
+      const doc = create(true)
+      doc.enablePatches(true)
+      let mat : any = doc.materialize("/")
+      doc.putObject("/", "text", "abcdefghij")
+      assert.throws(() => {
+        doc.applyPatches(mat, {}, (patches, info) => {
+          throw new RangeError("hello world")
+        })
+      }, /RangeError: hello world/)
+    })
+
+    it('patch callback has correct patch info', () => {
+      const doc = create(true)
+      let mat : any = doc.materialize("/")
+      doc.putObject("/", "text", "abcdefghij")
+
+      let before = doc.materialize("/")
+      let from = doc.getHeads()
+
+      doc.enablePatches(true)
+      doc.splice("/text", 2, 2, "00")
+
+      let after = doc.materialize("/")
+      let to = doc.getHeads()
+
+      doc.applyPatches(mat, {}, (patches, info) => {
+        assert.deepEqual(info.before, before);
+        assert.deepEqual(info.after, after);
+        assert.deepEqual(info.from, from);
+        assert.deepEqual(info.to, to);
+      })
+    })
+
     it('can handle utf16 text', () => {
       const doc = create(true)
       doc.enablePatches(true)
diff --git a/rust/automerge/examples/watch.rs b/rust/automerge/examples/watch.rs
index 4cd8f4ea..c2b909ee 100644
--- a/rust/automerge/examples/watch.rs
+++ b/rust/automerge/examples/watch.rs
@@ -1,11 +1,11 @@
+use automerge::op_observer::HasPatches;
 use automerge::transaction::CommitOptions;
 use automerge::transaction::Transactable;
 use automerge::Automerge;
 use automerge::AutomergeError;
-use automerge::Patch;
-use automerge::ReadDoc;
 use automerge::VecOpObserver;
 use automerge::ROOT;
+use automerge::{Patch, PatchAction};
 
 fn main() {
     let mut doc = Automerge::new();
@@ -42,64 +42,52 @@ fn main() {
     get_changes(&doc, patches);
 }
 
-fn get_changes(doc: &Automerge, patches: Vec<Patch>) {
-    for patch in patches {
-        match patch {
-            Patch::Put {
-                obj, prop, value, ..
-            } => {
+fn get_changes(_doc: &Automerge, patches: Vec<Patch<char>>) {
+    for Patch { obj, path, action } in patches {
+        match action {
+            PatchAction::PutMap { key, value, .. } => {
                 println!(
                     "put {:?} at {:?} in obj {:?}, object path {:?}",
-                    value,
-                    prop,
-                    obj,
-                    doc.path_to_object(&obj)
+                    value, key, obj, path,
                 )
             }
-            Patch::Insert {
-                obj, index, value, ..
-            } => {
+            PatchAction::PutSeq { index, value, .. } => {
+                println!(
+                    "put {:?} at {:?} in obj {:?}, object path {:?}",
+                    value, index, obj, path,
+                )
+            }
+            PatchAction::Insert { index, values, .. } => {
                 println!(
                     "insert {:?} at {:?} in obj {:?}, object path {:?}",
-                    value,
-                    index,
-                    obj,
-                    doc.path_to_object(&obj)
+                    values, index, obj, path,
                 )
             }
-            Patch::Splice {
-                obj, index, value, ..
-            } => {
+            PatchAction::SpliceText { index, value, .. } => {
                 println!(
                     "splice '{:?}' at {:?} in obj {:?}, object path {:?}",
-                    value,
-                    index,
-                    obj,
-                    doc.path_to_object(&obj)
+                    value, index, obj, path,
                 )
             }
-            Patch::Increment {
-                obj, prop, value, ..
-            } => {
+            PatchAction::Increment { prop, value, .. } => {
                 println!(
                     "increment {:?} in obj {:?} by {:?}, object path {:?}",
-                    prop,
-                    obj,
-                    value,
-                    doc.path_to_object(&obj)
+                    prop, obj, value, path,
                 )
             }
-            Patch::Delete { obj, prop, .. } => println!(
+            PatchAction::DeleteMap { key, .. } => {
+                println!("delete {:?} in obj {:?}, object path {:?}", key, obj, path,)
+            }
+            PatchAction::DeleteSeq { index, .. } => println!(
                 "delete {:?} in obj {:?}, object path {:?}",
-                prop,
-                obj,
-                doc.path_to_object(&obj)
+                index, obj, path,
             ),
-            Patch::Expose { obj, prop, .. } => println!(
-                "expose {:?} in obj {:?}, object path {:?}",
-                prop,
-                obj,
-                doc.path_to_object(&obj)
+            PatchAction::Mark { marks } => {
+                println!("mark {:?} in obj {:?}, object path {:?}", marks, obj, path,)
+            }
+            PatchAction::Unmark { name, start, end } => println!(
+                "unmark {:?} from {} to {} in obj {:?}, object path {:?}",
+                name, start, end, obj, path,
             ),
         }
     }
diff --git a/rust/automerge/src/autocommit.rs b/rust/automerge/src/autocommit.rs
index ae28596e..a5685f48 100644
--- a/rust/automerge/src/autocommit.rs
+++ b/rust/automerge/src/autocommit.rs
@@ -1,6 +1,7 @@
 use std::ops::RangeBounds;
 
 use crate::exid::ExId;
+use crate::marks::{ExpandMark, Mark};
 use crate::op_observer::{BranchableObserver, OpObserver};
 use crate::sync::SyncDoc;
 use crate::transaction::{CommitOptions, Transactable};
@@ -280,6 +281,11 @@ impl<Obs: Observation> AutoCommitWithObs<Obs> {
         self.doc.import(s)
     }
 
+    #[doc(hidden)]
+    pub fn import_obj(&self, s: &str) -> Result<ExId, AutomergeError> {
+        self.doc.import_obj(s)
+    }
+
     #[doc(hidden)]
     pub fn dump(&mut self) {
         self.ensure_transaction_closed();
@@ -441,6 +447,18 @@ impl<Obs: Observation> ReadDoc for AutoCommitWithObs<Obs> {
         self.doc.object_type(obj)
     }
 
+    fn marks<O: AsRef<ExId>>(&self, obj: O) -> Result<Vec<Mark<'_>>, AutomergeError> {
+        self.doc.marks(obj)
+    }
+
+    fn marks_at<O: AsRef<ExId>>(
+        &self,
+        obj: O,
+        heads: &[ChangeHash],
+    ) -> Result<Vec<Mark<'_>>, AutomergeError> {
+        self.doc.marks_at(obj, heads)
+    }
+
     fn text<O: AsRef<ExId>>(&self, obj: O) -> Result<String, AutomergeError> {
         self.doc.text(obj)
     }
@@ -621,6 +639,42 @@ impl<Obs: Observation> Transactable for AutoCommitWithObs<Obs> {
         )
     }
 
+    fn mark<O: AsRef<ExId>>(
+        &mut self,
+        obj: O,
+        mark: Mark<'_>,
+        expand: ExpandMark,
+    ) -> Result<(), AutomergeError> {
+        self.ensure_transaction_open();
+        let (current, tx) = self.transaction.as_mut().unwrap();
+        tx.mark(
+            &mut self.doc,
+            current.observer(),
+            obj.as_ref(),
+            mark,
+            expand,
+        )
+    }
+
+    fn unmark<O: AsRef<ExId>>(
+        &mut self,
+        obj: O,
+        key: &str,
+        start: usize,
+        end: usize,
+    ) -> Result<(), AutomergeError> {
+        self.ensure_transaction_open();
+        let (current, tx) = self.transaction.as_mut().unwrap();
+        tx.unmark(
+            &mut self.doc,
+            current.observer(),
+            obj.as_ref(),
+            key,
+            start,
+            end,
+        )
+    }
+
     fn base_heads(&self) -> Vec<ChangeHash> {
         self.doc.get_heads()
     }
diff --git a/rust/automerge/src/automerge.rs b/rust/automerge/src/automerge.rs
index 0dd82253..c0fcfe28 100644
--- a/rust/automerge/src/automerge.rs
+++ b/rust/automerge/src/automerge.rs
@@ -4,10 +4,13 @@ use std::fmt::Debug;
 use std::num::NonZeroU64;
 use std::ops::RangeBounds;
 
+use itertools::Itertools;
+
 use crate::change_graph::ChangeGraph;
 use crate::columnar::Key as EncodedKey;
 use crate::exid::ExId;
 use crate::keys::Keys;
+use crate::marks::{Mark, MarkStateMachine};
 use crate::op_observer::{BranchableObserver, OpObserver};
 use crate::op_set::OpSet;
 use crate::parents::Parents;
@@ -16,14 +19,13 @@ use crate::transaction::{
     self, CommitOptions, Failure, Observed, Success, Transaction, TransactionArgs, UnObserved,
 };
 use crate::types::{
-    ActorId, ChangeHash, Clock, ElemId, Export, Exportable, Key, ListEncoding, ObjId, Op, OpId,
-    OpType, ScalarValue, TextEncoding, Value,
+    ActorId, ChangeHash, Clock, ElemId, Export, Exportable, Key, ListEncoding, MarkData, ObjId, Op,
+    OpId, OpType, ScalarValue, TextEncoding, Value,
 };
 use crate::{
     query, AutomergeError, Change, KeysAt, ListRange, ListRangeAt, MapRange, MapRangeAt, ObjType,
     Prop, ReadDoc, Values,
 };
-use serde::Serialize;
 
 mod current_state;
 
@@ -400,11 +402,23 @@ impl Automerge {
     pub(crate) fn exid_to_obj(&self, id: &ExId) -> Result<(ObjId, ObjType), AutomergeError> {
         match id {
             ExId::Root => Ok((ObjId::root(), ObjType::Map)),
+            ExId::Id(..) => {
+                let obj = ObjId(self.exid_to_opid(id)?);
+                if let Some(obj_type) = self.ops.object_type(&obj) {
+                    Ok((obj, obj_type))
+                } else {
+                    Err(AutomergeError::NotAnObject)
+                }
+            }
+        }
+    }
+
+    pub(crate) fn exid_to_opid(&self, id: &ExId) -> Result<OpId, AutomergeError> {
+        match id {
+            ExId::Root => Err(AutomergeError::Fail),
             ExId::Id(ctr, actor, idx) => {
-                // do a direct get here b/c this could be foriegn and not be within the array
-                // bounds
-                let obj = if self.ops.m.actors.cache.get(*idx) == Some(actor) {
-                    ObjId(OpId::new(*ctr, *idx))
+                if self.ops.m.actors.cache.get(*idx) == Some(actor) {
+                    Ok(OpId::new(*ctr, *idx))
                 } else {
                     // FIXME - make a real error
                     let idx = self
@@ -413,12 +427,7 @@ impl Automerge {
                         .actors
                         .lookup(actor)
                         .ok_or(AutomergeError::Fail)?;
-                    ObjId(OpId::new(*ctr, idx))
-                };
-                if let Some(obj_type) = self.ops.object_type(&obj) {
-                    Ok((obj, obj_type))
-                } else {
-                    Err(AutomergeError::NotAnObject)
+                    Ok(OpId::new(*ctr, idx))
                 }
             }
         }
@@ -723,7 +732,12 @@ impl Automerge {
                     obj,
                     Op {
                         id,
-                        action: OpType::from_action_and_value(c.action, c.val),
+                        action: OpType::from_action_and_value(
+                            c.action,
+                            c.val,
+                            c.mark_name,
+                            c.expand,
+                        ),
                         key,
                         succ: Default::default(),
                         pred,
@@ -913,8 +927,21 @@ impl Automerge {
 
     #[doc(hidden)]
     pub fn import(&self, s: &str) -> Result<(ExId, ObjType), AutomergeError> {
-        if s == "_root" {
+        let obj = self.import_obj(s)?;
+        if obj == ExId::Root {
             Ok((ExId::Root, ObjType::Map))
+        } else {
+            let obj_type = self
+                .object_type(&obj)
+                .map_err(|_| AutomergeError::InvalidObjId(s.to_owned()))?;
+            Ok((obj, obj_type))
+        }
+    }
+
+    #[doc(hidden)]
+    pub fn import_obj(&self, s: &str) -> Result<ExId, AutomergeError> {
+        if s == "_root" {
+            Ok(ExId::Root)
         } else {
             let n = s
                 .find('@')
@@ -930,10 +957,7 @@ impl Automerge {
                 .lookup(&actor)
                 .ok_or_else(|| AutomergeError::InvalidObjId(s.to_owned()))?;
             let obj = ExId::Id(counter, self.ops.m.actors.cache[actor].clone(), actor);
-            let obj_type = self
-                .object_type(&obj)
-                .map_err(|_| AutomergeError::InvalidObjId(s.to_owned()))?;
-            Ok((obj, obj_type))
+            Ok(obj)
         }
     }
 
@@ -967,6 +991,10 @@ impl Automerge {
                 OpType::Make(obj) => format!("make({})", obj),
                 OpType::Increment(obj) => format!("inc({})", obj),
                 OpType::Delete => format!("del{}", 0),
+                OpType::MarkBegin(_, MarkData { name, value }) => {
+                    format!("mark({},{})", name, value)
+                }
+                OpType::MarkEnd(_) => "/mark".to_string(),
             };
             let pred: Vec<_> = op.pred.iter().map(|id| self.to_string(*id)).collect();
             let succ: Vec<_> = op.succ.into_iter().map(|id| self.to_string(*id)).collect();
@@ -1045,11 +1073,18 @@ impl Automerge {
         };
 
         if op.insert {
-            if obj_type == Some(ObjType::Text) {
+            if op.is_mark() {
+                if let OpType::MarkEnd(_) = op.action {
+                    let q = self
+                        .ops
+                        .search(obj, query::SeekMark::new(op.id.prev(), pos, encoding));
+                    observer.mark(self, ex_obj, q.marks.into_iter());
+                }
+            } else if obj_type == Some(ObjType::Text) {
                 observer.splice_text(self, ex_obj, seen, op.to_str());
             } else {
                 let value = (op.value(), self.ops.id_to_exid(op.id));
-                observer.insert(self, ex_obj, seen, value);
+                observer.insert(self, ex_obj, seen, value, false);
             }
         } else if op.is_delete() {
             if let Some(winner) = &values.last() {
@@ -1081,7 +1116,7 @@ impl Automerge {
                 .unwrap_or(false);
             let value = (op.value(), self.ops.id_to_exid(op.id));
             if op.is_list_op() && !had_value_before {
-                observer.insert(self, ex_obj, seen, value);
+                observer.insert(self, ex_obj, seen, value, false);
             } else if just_conflict {
                 observer.flag_conflict(self, ex_obj, key);
             } else {
@@ -1309,6 +1344,64 @@ impl ReadDoc for Automerge {
         Ok(buffer)
     }
 
+    fn marks<O: AsRef<ExId>>(&self, obj: O) -> Result<Vec<Mark<'_>>, AutomergeError> {
+        let (obj, obj_type) = self.exid_to_obj(obj.as_ref())?;
+        let encoding = ListEncoding::new(obj_type, self.text_encoding);
+        let ops_by_key = self.ops().iter_ops(&obj).group_by(|o| o.elemid_or_key());
+        let mut pos = 0;
+        let mut marks = MarkStateMachine::default();
+
+        Ok(ops_by_key
+            .into_iter()
+            .filter_map(|(_key, key_ops)| {
+                key_ops
+                    .filter(|o| o.visible_or_mark())
+                    .last()
+                    .and_then(|o| match &o.action {
+                        OpType::Make(_) | OpType::Put(_) => {
+                            pos += o.width(encoding);
+                            None
+                        }
+                        OpType::MarkBegin(_, data) => marks.mark_begin(o.id, pos, data, self),
+                        OpType::MarkEnd(_) => marks.mark_end(o.id, pos, self),
+                        OpType::Increment(_) | OpType::Delete => None,
+                    })
+            })
+            .collect())
+    }
+
+    fn marks_at<O: AsRef<ExId>>(
+        &self,
+        obj: O,
+        heads: &[ChangeHash],
+    ) -> Result<Vec<Mark<'_>>, AutomergeError> {
+        let (obj, obj_type) = self.exid_to_obj(obj.as_ref())?;
+        let clock = self.clock_at(heads);
+        let encoding = ListEncoding::new(obj_type, self.text_encoding);
+        let ops_by_key = self.ops().iter_ops(&obj).group_by(|o| o.elemid_or_key());
+        let mut window = query::VisWindow::default();
+        let mut pos = 0;
+        let mut marks = MarkStateMachine::default();
+
+        Ok(ops_by_key
+            .into_iter()
+            .filter_map(|(_key, key_ops)| {
+                key_ops
+                    .filter(|o| window.visible_at(o, pos, &clock))
+                    .last()
+                    .and_then(|o| match &o.action {
+                        OpType::Make(_) | OpType::Put(_) => {
+                            pos += o.width(encoding);
+                            None
+                        }
+                        OpType::MarkBegin(_, data) => marks.mark_begin(o.id, pos, data, self),
+                        OpType::MarkEnd(_) => marks.mark_end(o.id, pos, self),
+                        OpType::Increment(_) | OpType::Delete => None,
+                    })
+            })
+            .collect())
+    }
+
     fn get<O: AsRef<ExId>, P: Into<Prop>>(
         &self,
         obj: O,
@@ -1439,14 +1532,3 @@ impl Default for Automerge {
         Self::new()
     }
 }
-
-#[derive(Serialize, Debug, Clone, PartialEq)]
-pub(crate) struct SpanInfo {
-    pub(crate) id: ExId,
-    pub(crate) time: i64,
-    pub(crate) start: usize,
-    pub(crate) end: usize,
-    #[serde(rename = "type")]
-    pub(crate) span_type: String,
-    pub(crate) value: ScalarValue,
-}
diff --git a/rust/automerge/src/automerge/current_state.rs b/rust/automerge/src/automerge/current_state.rs
index 3f7f4afc..bb741a76 100644
--- a/rust/automerge/src/automerge/current_state.rs
+++ b/rust/automerge/src/automerge/current_state.rs
@@ -1,12 +1,27 @@
-use std::{borrow::Cow, collections::HashSet, iter::Peekable};
+use std::borrow::Cow;
 
 use itertools::Itertools;
 
 use crate::{
-    types::{ElemId, Key, ListEncoding, ObjId, Op, OpId},
-    ObjType, OpObserver, OpType, ScalarValue, Value,
+    marks::{Mark, MarkStateMachine},
+    types::{Key, ListEncoding, ObjId, Op, OpId, Prop},
+    Automerge, ObjType, OpObserver, OpType, Value,
 };
 
+#[derive(Debug, Default)]
+struct TextState<'a> {
+    text: String,
+    len: usize,
+    marks: MarkStateMachine<'a>,
+    finished: Vec<Mark<'a>>,
+}
+
+struct Put<'a> {
+    value: Value<'a>,
+    key: Key,
+    id: OpId,
+}
+
 /// Traverse the "current" state of the document, notifying `observer`
 ///
 /// The "current" state of the document is the set of visible operations. This function will
@@ -17,7 +32,8 @@ use crate::{
 ///
 /// Due to only notifying of visible operations the observer will only be called with `put`,
 /// `insert`, and `splice`, operations.
-pub(super) fn observe_current_state<O: OpObserver>(doc: &crate::Automerge, observer: &mut O) {
+
+pub(crate) fn observe_current_state<O: OpObserver>(doc: &Automerge, observer: &mut O) {
     // The OpSet already exposes operations in the order they appear in the document.
     // `OpSet::iter_objs` iterates over the objects in causal order, this means that parent objects
     // will always appear before their children. Furthermore, the operations within each object are
@@ -26,321 +42,170 @@ pub(super) fn observe_current_state<O: OpObserver>(doc: &crate::Automerge, obser
     // Effectively then we iterate over each object, then we group the operations in the object by
     // key and for each key find the visible operations for that key. Then we notify the observer
     // for each of those visible operations.
-    let mut visible_objs = HashSet::new();
-    visible_objs.insert(ObjId::root());
     for (obj, typ, ops) in doc.ops().iter_objs() {
-        if !visible_objs.contains(obj) {
-            continue;
-        }
-        let ops_by_key = ops.group_by(|o| o.key);
-        let actions = ops_by_key
-            .into_iter()
-            .flat_map(|(key, key_ops)| key_actions(key, key_ops));
         if typ == ObjType::Text && !observer.text_as_seq() {
-            track_new_objs_and_notify(
-                &mut visible_objs,
-                doc,
-                obj,
-                typ,
-                observer,
-                text_actions(actions),
-            )
-        } else if typ == ObjType::List {
-            track_new_objs_and_notify(
-                &mut visible_objs,
-                doc,
-                obj,
-                typ,
-                observer,
-                list_actions(actions),
-            )
+            observe_text(doc, observer, obj, ops)
+        } else if typ.is_sequence() {
+            observe_list(doc, observer, obj, ops);
         } else {
-            track_new_objs_and_notify(&mut visible_objs, doc, obj, typ, observer, actions)
+            observe_map(doc, observer, obj, ops);
         }
     }
 }
 
-fn track_new_objs_and_notify<N: Action, I: Iterator<Item = N>, O: OpObserver>(
-    visible_objs: &mut HashSet<ObjId>,
-    doc: &crate::Automerge,
-    obj: &ObjId,
-    typ: ObjType,
+fn observe_text<'a, I: Iterator<Item = &'a Op>, O: OpObserver>(
+    doc: &'a Automerge,
     observer: &mut O,
-    actions: I,
+    obj: &ObjId,
+    ops: I,
 ) {
     let exid = doc.id_to_exid(obj.0);
-    for action in actions {
-        if let Some(obj) = action.made_object() {
-            visible_objs.insert(obj);
-        }
-        action.notify_observer(doc, &exid, obj, typ, observer);
-    }
-}
-
-trait Action {
-    /// Notify an observer of whatever this action does
-    fn notify_observer<O: OpObserver>(
-        self,
-        doc: &crate::Automerge,
-        exid: &crate::ObjId,
-        obj: &ObjId,
-        typ: ObjType,
-        observer: &mut O,
-    );
-
-    /// If this action created an object, return the ID of that object
-    fn made_object(&self) -> Option<ObjId>;
-}
-
-fn key_actions<'a, I: Iterator<Item = &'a Op>>(
-    key: Key,
-    key_ops: I,
-) -> impl Iterator<Item = SimpleAction<'a>> {
-    #[derive(Clone)]
-    enum CurrentOp<'a> {
-        Put {
-            value: Value<'a>,
-            id: OpId,
-            conflicted: bool,
-        },
-        Insert(Value<'a>, OpId),
-    }
-    let current_ops = key_ops
-        .filter(|o| o.visible())
-        .filter_map(|o| match o.action {
-            OpType::Make(obj_type) => {
-                let value = Value::Object(obj_type);
-                if o.insert {
-                    Some(CurrentOp::Insert(value, o.id))
-                } else {
-                    Some(CurrentOp::Put {
-                        value,
-                        id: o.id,
-                        conflicted: false,
-                    })
-                }
-            }
-            OpType::Put(ref value) => {
-                let value = Value::Scalar(Cow::Borrowed(value));
-                if o.insert {
-                    Some(CurrentOp::Insert(value, o.id))
-                } else {
-                    Some(CurrentOp::Put {
-                        value,
-                        id: o.id,
-                        conflicted: false,
-                    })
-                }
-            }
-            _ => None,
-        });
-    current_ops
-        .coalesce(|previous, current| match (previous, current) {
-            (CurrentOp::Put { .. }, CurrentOp::Put { value, id, .. }) => Ok(CurrentOp::Put {
-                value,
-                id,
-                conflicted: true,
-            }),
-            (previous, current) => Err((previous, current)),
-        })
-        .map(move |op| match op {
-            CurrentOp::Put {
-                value,
-                id,
-                conflicted,
-            } => SimpleAction::Put {
-                prop: key,
-                tagged_value: (value, id),
-                conflict: conflicted,
-            },
-            CurrentOp::Insert(val, id) => SimpleAction::Insert {
-                elem_id: ElemId(id),
-                tagged_value: (val, id),
-            },
-        })
-}
-
-/// Either a "put" or "insert" action. i.e. not splicing for text values
-enum SimpleAction<'a> {
-    Put {
-        prop: Key,
-        tagged_value: (Value<'a>, OpId),
-        conflict: bool,
-    },
-    Insert {
-        elem_id: ElemId,
-        tagged_value: (Value<'a>, OpId),
-    },
-}
-
-impl<'a> Action for SimpleAction<'a> {
-    fn notify_observer<O: OpObserver>(
-        self,
-        doc: &crate::Automerge,
-        exid: &crate::ObjId,
-        obj: &ObjId,
-        typ: ObjType,
-        observer: &mut O,
-    ) {
-        let encoding = match typ {
-            ObjType::Text => ListEncoding::Text(doc.text_encoding()),
-            _ => ListEncoding::List,
-        };
-        match self {
-            Self::Put {
-                prop,
-                tagged_value,
-                conflict,
-            } => {
-                let tagged_value = (tagged_value.0, doc.id_to_exid(tagged_value.1));
-                let prop = doc.ops().export_key(*obj, prop, encoding).unwrap();
-                observer.put(doc, exid.clone(), prop, tagged_value, conflict);
-            }
-            Self::Insert {
-                elem_id,
-                tagged_value: (value, opid),
-            } => {
-                let index = doc
-                    .ops()
-                    .search(obj, crate::query::ElemIdPos::new(elem_id, encoding))
-                    .index()
-                    .unwrap();
-                let tagged_value = (value, doc.id_to_exid(opid));
-                observer.insert(doc, doc.id_to_exid(obj.0), index, tagged_value);
-            }
-        }
-    }
-
-    fn made_object(&self) -> Option<ObjId> {
-        match self {
-            Self::Put {
-                tagged_value: (Value::Object(_), id),
-                ..
-            } => Some((*id).into()),
-            Self::Insert {
-                tagged_value: (Value::Object(_), id),
-                ..
-            } => Some((*id).into()),
-            _ => None,
-        }
-    }
-}
-
-/// An `Action` which splices for text values
-enum TextAction<'a> {
-    Action(SimpleAction<'a>),
-    Splice { start: ElemId, chars: String },
-}
-
-impl<'a> Action for TextAction<'a> {
-    fn notify_observer<O: OpObserver>(
-        self,
-        doc: &crate::Automerge,
-        exid: &crate::ObjId,
-        obj: &ObjId,
-        typ: ObjType,
-        observer: &mut O,
-    ) {
-        match self {
-            Self::Action(action) => action.notify_observer(doc, exid, obj, typ, observer),
-            Self::Splice { start, chars } => {
-                let index = doc
-                    .ops()
-                    .search(
-                        obj,
-                        crate::query::ElemIdPos::new(
-                            start,
-                            ListEncoding::Text(doc.text_encoding()),
-                        ),
-                    )
-                    .index()
-                    .unwrap();
-                observer.splice_text(doc, doc.id_to_exid(obj.0), index, chars.as_str());
-            }
-        }
-    }
-
-    fn made_object(&self) -> Option<ObjId> {
-        match self {
-            Self::Action(action) => action.made_object(),
-            _ => None,
-        }
-    }
-}
-
-fn list_actions<'a, I: Iterator<Item = SimpleAction<'a>>>(
-    actions: I,
-) -> impl Iterator<Item = SimpleAction<'a>> {
-    actions.map(|a| match a {
-        SimpleAction::Put {
-            prop: Key::Seq(elem_id),
-            tagged_value,
-            ..
-        } => SimpleAction::Insert {
-            elem_id,
-            tagged_value,
-        },
-        a => a,
-    })
-}
-
-/// Condense consecutive `SimpleAction::Insert` actions into one `TextAction::Splice`
-fn text_actions<'a, I>(actions: I) -> impl Iterator<Item = TextAction<'a>>
-where
-    I: Iterator<Item = SimpleAction<'a>>,
-{
-    TextActions {
-        ops: actions.peekable(),
-    }
-}
-
-struct TextActions<'a, I: Iterator<Item = SimpleAction<'a>>> {
-    ops: Peekable<I>,
-}
-
-impl<'a, I: Iterator<Item = SimpleAction<'a>>> Iterator for TextActions<'a, I> {
-    type Item = TextAction<'a>;
-
-    fn next(&mut self) -> Option<Self::Item> {
-        if let Some(SimpleAction::Insert { .. }) = self.ops.peek() {
-            let (start, value) = match self.ops.next() {
-                Some(SimpleAction::Insert {
-                    tagged_value: (value, opid),
-                    ..
-                }) => (opid, value),
-                _ => unreachable!(),
-            };
-            let mut chars = match value {
-                Value::Scalar(Cow::Borrowed(ScalarValue::Str(s))) => s.to_string(),
-                _ => "\u{fffc}".to_string(),
-            };
-            while let Some(SimpleAction::Insert { .. }) = self.ops.peek() {
-                if let Some(SimpleAction::Insert {
-                    tagged_value: (value, _),
-                    ..
-                }) = self.ops.next()
-                {
-                    match value {
-                        Value::Scalar(Cow::Borrowed(ScalarValue::Str(s))) => chars.push_str(s),
-                        _ => chars.push('\u{fffc}'),
+    let ops_by_key = ops.group_by(|o| o.elemid_or_key());
+    let encoding = ListEncoding::Text(doc.text_encoding());
+    let state = TextState::default();
+    let state = ops_by_key
+        .into_iter()
+        .fold(state, |mut state, (_key, key_ops)| {
+            if let Some(o) = key_ops.filter(|o| o.visible_or_mark()).last() {
+                match &o.action {
+                    OpType::Make(_) | OpType::Put(_) => {
+                        state.text.push_str(o.to_str());
+                        state.len += o.width(encoding);
                     }
+                    OpType::MarkBegin(_, data) => {
+                        if let Some(mark) = state.marks.mark_begin(o.id, state.len, data, doc) {
+                            state.finished.push(mark);
+                        }
+                    }
+                    OpType::MarkEnd(_) => {
+                        if let Some(mark) = state.marks.mark_end(o.id, state.len, doc) {
+                            state.finished.push(mark);
+                        }
+                    }
+                    OpType::Increment(_) | OpType::Delete => {}
                 }
             }
-            Some(TextAction::Splice {
-                start: ElemId(start),
-                chars,
-            })
-        } else {
-            self.ops.next().map(TextAction::Action)
-        }
-    }
+            state
+        });
+    observer.splice_text(doc, exid.clone(), 0, state.text.as_str());
+    observer.mark(doc, exid, state.finished.into_iter());
+}
+
+fn observe_list<'a, I: Iterator<Item = &'a Op>, O: OpObserver>(
+    doc: &'a Automerge,
+    observer: &mut O,
+    obj: &ObjId,
+    ops: I,
+) {
+    let exid = doc.id_to_exid(obj.0);
+    let mut marks = MarkStateMachine::default();
+    let ops_by_key = ops.group_by(|o| o.elemid_or_key());
+    let mut len = 0;
+    let mut finished = Vec::new();
+    ops_by_key
+        .into_iter()
+        .filter_map(|(_key, key_ops)| {
+            key_ops
+                .filter(|o| o.visible_or_mark())
+                .filter_map(|o| match &o.action {
+                    OpType::Make(obj_type) => Some((Value::Object(*obj_type), o.id)),
+                    OpType::Put(value) => Some((Value::Scalar(Cow::Borrowed(value)), o.id)),
+                    OpType::MarkBegin(_, data) => {
+                        if let Some(mark) = marks.mark_begin(o.id, len, data, doc) {
+                            // side effect
+                            finished.push(mark)
+                        }
+                        None
+                    }
+                    OpType::MarkEnd(_) => {
+                        if let Some(mark) = marks.mark_end(o.id, len, doc) {
+                            // side effect
+                            finished.push(mark)
+                        }
+                        None
+                    }
+                    _ => None,
+                })
+                .enumerate()
+                .last()
+                .map(|value| {
+                    let pos = len;
+                    len += 1; // increment - side effect
+                    (pos, value)
+                })
+        })
+        .for_each(|(index, (val_enum, (value, opid)))| {
+            let tagged_value = (value, doc.id_to_exid(opid));
+            let conflict = val_enum > 0;
+            observer.insert(doc, exid.clone(), index, tagged_value, conflict);
+        });
+    observer.mark(doc, exid, finished.into_iter());
+}
+
+fn observe_map_key<'a, I: Iterator<Item = &'a Op>>(
+    (key, key_ops): (Key, I),
+) -> Option<(usize, Put<'a>)> {
+    key_ops
+        .filter(|o| o.visible())
+        .filter_map(|o| match &o.action {
+            OpType::Make(obj_type) => {
+                let value = Value::Object(*obj_type);
+                Some(Put {
+                    value,
+                    key,
+                    id: o.id,
+                })
+            }
+            OpType::Put(value) => {
+                let value = Value::Scalar(Cow::Borrowed(value));
+                Some(Put {
+                    value,
+                    key,
+                    id: o.id,
+                })
+            }
+            _ => None,
+        })
+        .enumerate()
+        .last()
+}
+
+fn observe_map<'a, I: Iterator<Item = &'a Op>, O: OpObserver>(
+    doc: &'a Automerge,
+    observer: &mut O,
+    obj: &ObjId,
+    ops: I,
+) {
+    let exid = doc.id_to_exid(obj.0);
+    let ops_by_key = ops.group_by(|o| o.key);
+    ops_by_key
+        .into_iter()
+        .filter_map(observe_map_key)
+        .filter_map(|(i, put)| {
+            let tagged_value = (put.value, doc.id_to_exid(put.id));
+            let prop = doc
+                .ops()
+                .m
+                .props
+                .safe_get(put.key.prop_index()?)
+                .map(|s| Prop::Map(s.to_string()))?;
+            let conflict = i > 0;
+            Some((tagged_value, prop, conflict))
+        })
+        .for_each(|(tagged_value, prop, conflict)| {
+            observer.put(doc, exid.clone(), prop, tagged_value, conflict);
+        });
 }
 
 #[cfg(test)]
 mod tests {
     use std::{borrow::Cow, fs};
 
-    use crate::{transaction::Transactable, Automerge, ObjType, OpObserver, Prop, ReadDoc, Value};
+    use crate::{
+        marks::Mark, transaction::Transactable, Automerge, ObjType, OpObserver, Prop, ReadDoc,
+        Value,
+    };
+    //use crate::{transaction::Transactable, Automerge, ObjType, OpObserver, Prop, ReadDoc, Value};
 
     // Observer ops often carry a "tagged value", which is a value and the OpID of the op which
     // created that value. For a lot of values (i.e. any scalar value) we don't care about the
@@ -490,6 +355,7 @@ mod tests {
             objid: crate::ObjId,
             index: usize,
             tagged_value: (crate::Value<'_>, crate::ObjId),
+            _conflict: bool,
         ) {
             self.ops.push(ObserverCall::Insert {
                 obj: objid,
@@ -566,6 +432,24 @@ mod tests {
         fn text_as_seq(&self) -> bool {
             self.text_as_seq
         }
+
+        fn mark<'a, R: ReadDoc, M: Iterator<Item = Mark<'a>>>(
+            &mut self,
+            _doc: &R,
+            _objid: crate::ObjId,
+            _mark: M,
+        ) {
+        }
+
+        fn unmark<R: ReadDoc>(
+            &mut self,
+            _doc: &R,
+            _objid: crate::ObjId,
+            _name: &str,
+            _start: usize,
+            _end: usize,
+        ) {
+        }
     }
 
     #[test]
diff --git a/rust/automerge/src/automerge/tests.rs b/rust/automerge/src/automerge/tests.rs
index 3511c4ed..e2863c3f 100644
--- a/rust/automerge/src/automerge/tests.rs
+++ b/rust/automerge/src/automerge/tests.rs
@@ -7,6 +7,9 @@ use crate::transaction::Transactable;
 use crate::*;
 use std::convert::TryInto;
 
+use crate::op_observer::HasPatches;
+use test_log::test;
+
 #[test]
 fn insert_op() -> Result<(), AutomergeError> {
     let mut doc = Automerge::new();
@@ -1479,15 +1482,18 @@ fn observe_counter_change_application_overwrite() {
 
     assert_eq!(
         doc3.observer().take_patches(),
-        vec![Patch::Put {
+        vec![Patch {
             obj: ExId::Root,
             path: vec![],
-            prop: Prop::Map("counter".into()),
-            value: (
-                ScalarValue::Str("mystring".into()).into(),
-                ExId::Id(2, doc2.get_actor().clone(), 1)
-            ),
-            conflict: false
+            action: PatchAction::PutMap {
+                key: "counter".into(),
+                value: (
+                    ScalarValue::Str("mystring".into()).into(),
+                    ExId::Id(2, doc2.get_actor().clone(), 1)
+                ),
+                conflict: false,
+                expose: false
+            }
         }]
     );
 
@@ -1514,29 +1520,29 @@ fn observe_counter_change_application() {
     new_doc.observer().take_patches();
     new_doc.apply_changes(changes).unwrap();
     assert_eq!(
-        new_doc.observer().take_patches(),
+        new_doc
+            .observer()
+            .take_patches()
+            .into_iter()
+            .map(|p| p.action)
+            .collect::<Vec<_>>(),
         vec![
-            Patch::Put {
-                obj: ExId::Root,
-                path: vec![],
-                prop: Prop::Map("counter".into()),
+            PatchAction::PutMap {
+                key: "counter".into(),
                 value: (
                     ScalarValue::counter(1).into(),
                     ExId::Id(1, doc.get_actor().clone(), 0)
                 ),
-                conflict: false
+                conflict: false,
+                expose: false,
             },
-            Patch::Increment {
-                obj: ExId::Root,
-                path: vec![],
+            PatchAction::Increment {
                 prop: Prop::Map("counter".into()),
-                value: (2, ExId::Id(2, doc.get_actor().clone(), 0)),
+                value: 2,
             },
-            Patch::Increment {
-                obj: ExId::Root,
-                path: vec![],
+            PatchAction::Increment {
                 prop: Prop::Map("counter".into()),
-                value: (5, ExId::Id(3, doc.get_actor().clone(), 0)),
+                value: 5,
             }
         ]
     );
diff --git a/rust/automerge/src/change.rs b/rust/automerge/src/change.rs
index be467a84..a6ab112a 100644
--- a/rust/automerge/src/change.rs
+++ b/rust/automerge/src/change.rs
@@ -255,6 +255,18 @@ mod convert_expanded {
                 None => Cow::Owned(ScalarValue::Null),
             }
         }
+
+        fn expand(&self) -> bool {
+            self.action.expand()
+        }
+
+        fn mark_name(&self) -> Option<Cow<'a, smol_str::SmolStr>> {
+            if let legacy::OpType::MarkBegin(legacy::MarkData { name, .. }) = &self.action {
+                Some(Cow::Borrowed(name))
+            } else {
+                None
+            }
+        }
     }
 
     impl<'a> convert::OpId<&'a ActorId> for &'a legacy::OpId {
@@ -278,7 +290,12 @@ impl From<&Change> for crate::ExpandedChange {
         let operations = c
             .iter_ops()
             .map(|o| crate::legacy::Op {
-                action: crate::types::OpType::from_action_and_value(o.action, o.val),
+                action: crate::legacy::OpType::from_parts(crate::legacy::OpTypeParts {
+                    action: o.action,
+                    value: o.val,
+                    expand: o.expand,
+                    mark_name: o.mark_name,
+                }),
                 insert: o.insert,
                 key: match o.key {
                     StoredKey::Elem(e) if e.is_head() => {
diff --git a/rust/automerge/src/columnar/column_range.rs b/rust/automerge/src/columnar/column_range.rs
index 5762ed14..709cbe9a 100644
--- a/rust/automerge/src/columnar/column_range.rs
+++ b/rust/automerge/src/columnar/column_range.rs
@@ -3,7 +3,7 @@ pub(crate) use rle::RleRange;
 mod delta;
 pub(crate) use delta::DeltaRange;
 mod boolean;
-pub(crate) use boolean::BooleanRange;
+pub(crate) use boolean::{BooleanRange, MaybeBooleanRange};
 mod raw;
 pub(crate) use raw::RawRange;
 mod opid;
diff --git a/rust/automerge/src/columnar/column_range/boolean.rs b/rust/automerge/src/columnar/column_range/boolean.rs
index 3cefaf0d..b2530462 100644
--- a/rust/automerge/src/columnar/column_range/boolean.rs
+++ b/rust/automerge/src/columnar/column_range/boolean.rs
@@ -1,6 +1,8 @@
 use std::{borrow::Cow, ops::Range};
 
-use crate::columnar::encoding::{BooleanDecoder, BooleanEncoder};
+use crate::columnar::encoding::{
+    BooleanDecoder, BooleanEncoder, MaybeBooleanDecoder, MaybeBooleanEncoder,
+};
 
 #[derive(Clone, Debug, PartialEq)]
 pub(crate) struct BooleanRange(Range<usize>);
@@ -38,3 +40,44 @@ impl From<BooleanRange> for Range<usize> {
         r.0
     }
 }
+
+#[derive(Clone, Debug, PartialEq)]
+pub struct MaybeBooleanRange(Range<usize>);
+
+impl MaybeBooleanRange {
+    pub(crate) fn decoder<'a>(&self, data: &'a [u8]) -> MaybeBooleanDecoder<'a> {
+        MaybeBooleanDecoder::from(Cow::Borrowed(&data[self.0.clone()]))
+    }
+
+    pub(crate) fn encode<I: Iterator<Item = bool>>(items: I, out: &mut Vec<u8>) -> Self {
+        let start = out.len();
+        let mut encoder = MaybeBooleanEncoder::from_sink(out);
+        for i in items {
+            encoder.append(i);
+        }
+        let (_, len) = encoder.finish();
+        (start..(start + len)).into()
+    }
+
+    pub(crate) fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+}
+
+impl AsRef<Range<usize>> for MaybeBooleanRange {
+    fn as_ref(&self) -> &Range<usize> {
+        &self.0
+    }
+}
+
+impl From<Range<usize>> for MaybeBooleanRange {
+    fn from(r: Range<usize>) -> MaybeBooleanRange {
+        MaybeBooleanRange(r)
+    }
+}
+
+impl From<MaybeBooleanRange> for Range<usize> {
+    fn from(r: MaybeBooleanRange) -> Range<usize> {
+        r.0
+    }
+}
diff --git a/rust/automerge/src/columnar/encoding.rs b/rust/automerge/src/columnar/encoding.rs
index c9435448..e15600d7 100644
--- a/rust/automerge/src/columnar/encoding.rs
+++ b/rust/automerge/src/columnar/encoding.rs
@@ -4,7 +4,9 @@ pub(crate) use raw::{RawDecoder, RawEncoder};
 mod rle;
 pub(crate) use rle::{RleDecoder, RleEncoder};
 mod boolean;
-pub(crate) use boolean::{BooleanDecoder, BooleanEncoder};
+pub(crate) use boolean::{
+    BooleanDecoder, BooleanEncoder, MaybeBooleanDecoder, MaybeBooleanEncoder,
+};
 mod delta;
 pub(crate) use delta::{DeltaDecoder, DeltaEncoder};
 pub(crate) mod leb128;
diff --git a/rust/automerge/src/columnar/encoding/boolean.rs b/rust/automerge/src/columnar/encoding/boolean.rs
index 26cb1838..00f27c16 100644
--- a/rust/automerge/src/columnar/encoding/boolean.rs
+++ b/rust/automerge/src/columnar/encoding/boolean.rs
@@ -100,6 +100,72 @@ impl<'a> Iterator for BooleanDecoder<'a> {
     }
 }
 
+/// Like a `BooleanEncoder` but if all the values in the column are `false` then will return an
+/// empty range rather than a range with `count` false values.
+pub(crate) struct MaybeBooleanEncoder<S> {
+    encoder: BooleanEncoder<S>,
+    all_false: bool,
+}
+
+impl MaybeBooleanEncoder<Vec<u8>> {
+    pub(crate) fn new() -> MaybeBooleanEncoder<Vec<u8>> {
+        MaybeBooleanEncoder::from_sink(Vec::new())
+    }
+}
+
+impl<S: Sink> MaybeBooleanEncoder<S> {
+    pub(crate) fn from_sink(buf: S) -> MaybeBooleanEncoder<S> {
+        MaybeBooleanEncoder {
+            encoder: BooleanEncoder::from_sink(buf),
+            all_false: true,
+        }
+    }
+
+    pub(crate) fn append(&mut self, value: bool) {
+        if value {
+            self.all_false = false;
+        }
+        self.encoder.append(value);
+    }
+
+    pub(crate) fn finish(self) -> (S, usize) {
+        if self.all_false {
+            (self.encoder.buf, 0)
+        } else {
+            self.encoder.finish()
+        }
+    }
+}
+
+/// Like a `BooleanDecoder` but if the underlying range is empty then just returns an infinite
+/// sequence of `None`
+#[derive(Clone, Debug)]
+pub(crate) struct MaybeBooleanDecoder<'a>(BooleanDecoder<'a>);
+
+impl<'a> From<Cow<'a, [u8]>> for MaybeBooleanDecoder<'a> {
+    fn from(bytes: Cow<'a, [u8]>) -> Self {
+        MaybeBooleanDecoder(BooleanDecoder::from(bytes))
+    }
+}
+
+impl<'a> From<&'a [u8]> for MaybeBooleanDecoder<'a> {
+    fn from(d: &'a [u8]) -> Self {
+        Cow::Borrowed(d).into()
+    }
+}
+
+impl<'a> Iterator for MaybeBooleanDecoder<'a> {
+    type Item = Result<Option<bool>, raw::Error>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.0.decoder.is_empty() {
+            None
+        } else {
+            self.0.next().transpose().map(Some).transpose()
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
diff --git a/rust/automerge/src/columnar/encoding/raw.rs b/rust/automerge/src/columnar/encoding/raw.rs
index b86443e5..5c46ef1e 100644
--- a/rust/automerge/src/columnar/encoding/raw.rs
+++ b/rust/automerge/src/columnar/encoding/raw.rs
@@ -59,6 +59,10 @@ impl<'a> RawDecoder<'a> {
     pub(crate) fn done(&self) -> bool {
         self.offset >= self.data.len()
     }
+
+    pub(crate) fn is_empty(&self) -> bool {
+        self.data.is_empty()
+    }
 }
 
 impl<'a> From<&'a [u8]> for RawDecoder<'a> {
diff --git a/rust/automerge/src/legacy/mod.rs b/rust/automerge/src/legacy/mod.rs
index 6e6acec5..d4fb1fd0 100644
--- a/rust/automerge/src/legacy/mod.rs
+++ b/rust/automerge/src/legacy/mod.rs
@@ -3,7 +3,7 @@ mod utility_impls;
 
 use std::num::NonZeroU64;
 
-pub(crate) use crate::types::{ActorId, ChangeHash, ObjType, OpType, ScalarValue};
+pub(crate) use crate::types::{ActorId, ChangeHash, ObjType, ScalarValue};
 pub(crate) use crate::value::DataType;
 
 use serde::{Deserialize, Serialize};
@@ -204,6 +204,96 @@ where
     }
 }
 
+pub(crate) struct OpTypeParts {
+    pub(crate) action: u64,
+    pub(crate) value: ScalarValue,
+    pub(crate) expand: bool,
+    pub(crate) mark_name: Option<smol_str::SmolStr>,
+}
+
+// Like `types::OpType` except using a String for mark names
+#[derive(PartialEq, Debug, Clone)]
+pub enum OpType {
+    Make(ObjType),
+    Delete,
+    Increment(i64),
+    Put(ScalarValue),
+    MarkBegin(MarkData),
+    MarkEnd(bool),
+}
+
+impl OpType {
+    /// Create a new legacy OpType
+    ///
+    /// This is really only meant to be used to convert from a crate::Change to a
+    /// crate::legacy::Change, so the arguments should all have been validated. Consequently it
+    /// does not return an error and instead panics on the following conditions
+    ///
+    /// # Panics
+    ///
+    /// * If The action index is unrecognized
+    /// * If the action index indicates that the value should be numeric but the value is not a
+    ///   number
+    pub(crate) fn from_parts(
+        OpTypeParts {
+            action,
+            value,
+            expand,
+            mark_name,
+        }: OpTypeParts,
+    ) -> Self {
+        match action {
+            0 => Self::Make(ObjType::Map),
+            1 => Self::Put(value),
+            2 => Self::Make(ObjType::List),
+            3 => Self::Delete,
+            4 => Self::Make(ObjType::Text),
+            5 => match value {
+                ScalarValue::Int(i) => Self::Increment(i),
+                ScalarValue::Uint(i) => Self::Increment(i as i64),
+                _ => panic!("non numeric value for integer action"),
+            },
+            6 => Self::Make(ObjType::Table),
+            7 => match mark_name {
+                Some(name) => Self::MarkBegin(MarkData {
+                    name,
+                    value,
+                    expand,
+                }),
+                None => Self::MarkEnd(expand),
+            },
+            other => panic!("unknown action type {}", other),
+        }
+    }
+
+    pub(crate) fn action_index(&self) -> u64 {
+        match self {
+            Self::Make(ObjType::Map) => 0,
+            Self::Put(_) => 1,
+            Self::Make(ObjType::List) => 2,
+            Self::Delete => 3,
+            Self::Make(ObjType::Text) => 4,
+            Self::Increment(_) => 5,
+            Self::Make(ObjType::Table) => 6,
+            Self::MarkBegin(_) | Self::MarkEnd(_) => 7,
+        }
+    }
+
+    pub(crate) fn expand(&self) -> bool {
+        matches!(
+            self,
+            Self::MarkBegin(MarkData { expand: true, .. }) | Self::MarkEnd(true)
+        )
+    }
+}
+
+#[derive(PartialEq, Debug, Clone)]
+pub struct MarkData {
+    pub name: smol_str::SmolStr,
+    pub value: ScalarValue,
+    pub expand: bool,
+}
+
 #[derive(PartialEq, Debug, Clone)]
 pub struct Op {
     pub action: OpType,
@@ -217,6 +307,7 @@ impl Op {
     pub fn primitive_value(&self) -> Option<ScalarValue> {
         match &self.action {
             OpType::Put(v) => Some(v.clone()),
+            OpType::MarkBegin(MarkData { value, .. }) => Some(value.clone()),
             OpType::Increment(i) => Some(ScalarValue::Int(*i)),
             _ => None,
         }
diff --git a/rust/automerge/src/legacy/serde_impls/op.rs b/rust/automerge/src/legacy/serde_impls/op.rs
index a3719fd6..d42879cd 100644
--- a/rust/automerge/src/legacy/serde_impls/op.rs
+++ b/rust/automerge/src/legacy/serde_impls/op.rs
@@ -5,7 +5,9 @@ use serde::{
 };
 
 use super::read_field;
-use crate::legacy::{DataType, Key, ObjType, ObjectId, Op, OpId, OpType, ScalarValue, SortedVec};
+use crate::legacy::{
+    DataType, Key, MarkData, ObjType, ObjectId, Op, OpId, OpType, ScalarValue, SortedVec,
+};
 
 impl Serialize for Op {
     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
@@ -50,6 +52,16 @@ impl Serialize for Op {
             OpType::Increment(n) => op.serialize_field("value", &n)?,
             OpType::Put(ScalarValue::Counter(c)) => op.serialize_field("value", &c.start)?,
             OpType::Put(value) => op.serialize_field("value", &value)?,
+            OpType::MarkBegin(MarkData {
+                name,
+                value,
+                expand,
+            }) => {
+                op.serialize_field("name", &name)?;
+                op.serialize_field("value", &value)?;
+                op.serialize_field("expand", &expand)?
+            }
+            OpType::MarkEnd(expand) => op.serialize_field("expand", &expand)?,
             _ => {}
         }
         op.serialize_field("pred", &self.pred)?;
@@ -71,6 +83,8 @@ pub(crate) enum RawOpType {
     Del,
     Inc,
     Set,
+    MarkBegin,
+    MarkEnd,
 }
 
 impl Serialize for RawOpType {
@@ -86,6 +100,8 @@ impl Serialize for RawOpType {
             RawOpType::Del => "del",
             RawOpType::Inc => "inc",
             RawOpType::Set => "set",
+            RawOpType::MarkBegin => "markBegin",
+            RawOpType::MarkEnd => "markEnd",
         };
         serializer.serialize_str(s)
     }
@@ -104,8 +120,8 @@ impl<'de> Deserialize<'de> for RawOpType {
             "del",
             "inc",
             "set",
-            "mark",
-            "unmark",
+            "markBegin",
+            "markEnd",
         ];
         // TODO: Probably more efficient to deserialize to a `&str`
         let raw_type = String::deserialize(deserializer)?;
@@ -117,6 +133,8 @@ impl<'de> Deserialize<'de> for RawOpType {
             "del" => Ok(RawOpType::Del),
             "inc" => Ok(RawOpType::Inc),
             "set" => Ok(RawOpType::Set),
+            "markBegin" => Ok(RawOpType::MarkBegin),
+            "markEnd" => Ok(RawOpType::MarkEnd),
             other => Err(Error::unknown_variant(other, VARIANTS)),
         }
     }
@@ -189,24 +207,7 @@ impl<'de> Deserialize<'de> for Op {
                     RawOpType::MakeList => OpType::Make(ObjType::List),
                     RawOpType::MakeText => OpType::Make(ObjType::Text),
                     RawOpType::Del => OpType::Delete,
-                    RawOpType::Set => {
-                        let value = if let Some(datatype) = datatype {
-                            let raw_value = value
-                                .ok_or_else(|| Error::missing_field("value"))?
-                                .unwrap_or(ScalarValue::Null);
-                            raw_value.as_datatype(datatype).map_err(|e| {
-                                Error::invalid_value(
-                                    Unexpected::Other(e.unexpected.as_str()),
-                                    &e.expected.as_str(),
-                                )
-                            })?
-                        } else {
-                            value
-                                .ok_or_else(|| Error::missing_field("value"))?
-                                .unwrap_or(ScalarValue::Null)
-                        };
-                        OpType::Put(value)
-                    }
+                    RawOpType::Set => OpType::Put(unwrap_value(value, datatype)?),
                     RawOpType::Inc => match value.flatten() {
                         Some(ScalarValue::Int(n)) => Ok(OpType::Increment(n)),
                         Some(ScalarValue::Uint(n)) => Ok(OpType::Increment(n as i64)),
@@ -230,6 +231,18 @@ impl<'de> Deserialize<'de> for Op {
                         }
                         None => Err(Error::missing_field("value")),
                     }?,
+                    RawOpType::MarkBegin => {
+                        let name = name.ok_or_else(|| Error::missing_field("name"))?;
+                        let name = smol_str::SmolStr::new(name);
+                        let expand = expand.unwrap_or(false);
+                        let value = unwrap_value(value, datatype)?;
+                        OpType::MarkBegin(MarkData {
+                            name,
+                            value,
+                            expand,
+                        })
+                    }
+                    RawOpType::MarkEnd => OpType::MarkEnd(expand.unwrap_or(false)),
                 };
                 Ok(Op {
                     action,
@@ -244,6 +257,27 @@ impl<'de> Deserialize<'de> for Op {
     }
 }
 
+fn unwrap_value<E: Error>(
+    value: Option<Option<ScalarValue>>,
+    datatype: Option<DataType>,
+) -> Result<ScalarValue, E> {
+    if let Some(datatype) = datatype {
+        let raw_value = value
+            .ok_or_else(|| Error::missing_field("value"))?
+            .unwrap_or(ScalarValue::Null);
+        raw_value.as_datatype(datatype).map_err(|e| {
+            Error::invalid_value(
+                Unexpected::Other(e.unexpected.as_str()),
+                &e.expected.as_str(),
+            )
+        })
+    } else {
+        Ok(value
+            .ok_or_else(|| Error::missing_field("value"))?
+            .unwrap_or(ScalarValue::Null))
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use std::str::FromStr;
diff --git a/rust/automerge/src/legacy/serde_impls/op_type.rs b/rust/automerge/src/legacy/serde_impls/op_type.rs
index b054bad7..32825119 100644
--- a/rust/automerge/src/legacy/serde_impls/op_type.rs
+++ b/rust/automerge/src/legacy/serde_impls/op_type.rs
@@ -1,7 +1,7 @@
 use serde::{Serialize, Serializer};
 
 use super::op::RawOpType;
-use crate::{ObjType, OpType};
+use crate::{legacy::OpType, ObjType};
 
 impl Serialize for OpType {
     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
@@ -18,6 +18,8 @@ impl Serialize for OpType {
             OpType::Delete => RawOpType::Del,
             OpType::Increment(_) => RawOpType::Inc,
             OpType::Put(_) => RawOpType::Set,
+            OpType::MarkBegin(_) => RawOpType::MarkBegin,
+            OpType::MarkEnd(_) => RawOpType::MarkEnd,
         };
         raw_type.serialize(serializer)
     }
diff --git a/rust/automerge/src/lib.rs b/rust/automerge/src/lib.rs
index cbb535af..ff6f6daa 100644
--- a/rust/automerge/src/lib.rs
+++ b/rust/automerge/src/lib.rs
@@ -258,12 +258,14 @@ mod list_range;
 mod list_range_at;
 mod map_range;
 mod map_range_at;
+pub mod marks;
 pub mod op_observer;
 mod op_set;
 mod op_tree;
 mod parents;
 mod query;
 mod read;
+mod sequence_tree;
 mod storage;
 pub mod sync;
 pub mod transaction;
@@ -288,9 +290,9 @@ pub use list_range::ListRange;
 pub use list_range_at::ListRangeAt;
 pub use map_range::MapRange;
 pub use map_range_at::MapRangeAt;
-pub use op_observer::OpObserver;
-pub use op_observer::Patch;
-pub use op_observer::VecOpObserver;
+pub use op_observer::{
+    OpObserver, Patch, PatchAction, ToggleObserver, VecOpObserver, VecOpObserver16,
+};
 pub use parents::{Parent, Parents};
 pub use read::ReadDoc;
 pub use types::{ActorId, ChangeHash, ObjType, OpType, ParseChangeHashError, Prop, TextEncoding};
diff --git a/rust/automerge/src/marks.rs b/rust/automerge/src/marks.rs
new file mode 100644
index 00000000..cb9520d9
--- /dev/null
+++ b/rust/automerge/src/marks.rs
@@ -0,0 +1,194 @@
+use smol_str::SmolStr;
+use std::fmt;
+use std::fmt::Display;
+
+use crate::types::OpId;
+use crate::value::ScalarValue;
+use crate::Automerge;
+use std::borrow::Cow;
+
+#[derive(Debug, Clone, PartialEq)]
+pub struct Mark<'a> {
+    pub start: usize,
+    pub end: usize,
+    pub(crate) data: Cow<'a, MarkData>,
+}
+
+impl<'a> Mark<'a> {
+    pub fn new<V: Into<ScalarValue>>(
+        name: String,
+        value: V,
+        start: usize,
+        end: usize,
+    ) -> Mark<'static> {
+        Mark {
+            data: Cow::Owned(MarkData {
+                name: name.into(),
+                value: value.into(),
+            }),
+            start,
+            end,
+        }
+    }
+
+    pub(crate) fn from_data(start: usize, end: usize, data: &MarkData) -> Mark<'_> {
+        Mark {
+            data: Cow::Borrowed(data),
+            start,
+            end,
+        }
+    }
+
+    pub fn into_owned(self) -> Mark<'static> {
+        Mark {
+            data: Cow::Owned(self.data.into_owned()),
+            start: self.start,
+            end: self.end,
+        }
+    }
+
+    pub fn name(&self) -> &str {
+        self.data.name.as_str()
+    }
+
+    pub fn value(&self) -> &ScalarValue {
+        &self.data.value
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Default)]
+pub(crate) struct MarkStateMachine<'a> {
+    state: Vec<(OpId, Mark<'a>)>,
+}
+
+impl<'a> MarkStateMachine<'a> {
+    pub(crate) fn mark_begin(
+        &mut self,
+        id: OpId,
+        pos: usize,
+        data: &'a MarkData,
+        doc: &Automerge,
+    ) -> Option<Mark<'a>> {
+        let mut result = None;
+        let index = self.find(id, doc).err()?;
+
+        let mut mark = Mark::from_data(pos, pos, data);
+
+        if let Some(above) = Self::mark_above(&self.state, index, &mark) {
+            if above.value() == mark.value() {
+                mark.start = above.start;
+            }
+        } else if let Some(below) = Self::mark_below(&mut self.state, index, &mark) {
+            if below.value() == mark.value() {
+                mark.start = below.start;
+            } else {
+                let mut m = below.clone();
+                m.end = pos;
+                if !m.value().is_null() {
+                    result = Some(m);
+                }
+            }
+        }
+
+        self.state.insert(index, (id, mark));
+
+        result
+    }
+
+    pub(crate) fn mark_end(&mut self, id: OpId, pos: usize, doc: &Automerge) -> Option<Mark<'a>> {
+        let mut result = None;
+        let index = self.find(id.prev(), doc).ok()?;
+
+        let mut mark = self.state.remove(index).1;
+        mark.end = pos;
+
+        if Self::mark_above(&self.state, index, &mark).is_none() {
+            match Self::mark_below(&mut self.state, index, &mark) {
+                Some(below) if below.value() == mark.value() => {}
+                Some(below) => {
+                    below.start = pos;
+                    if !mark.value().is_null() {
+                        result = Some(mark.clone());
+                    }
+                }
+                None => {
+                    if !mark.value().is_null() {
+                        result = Some(mark.clone());
+                    }
+                }
+            }
+        }
+
+        result
+    }
+
+    fn find(&self, target: OpId, doc: &Automerge) -> Result<usize, usize> {
+        let metadata = &doc.ops().m;
+        self.state
+            .binary_search_by(|probe| metadata.lamport_cmp(probe.0, target))
+    }
+
+    fn mark_above<'b>(
+        state: &'b [(OpId, Mark<'a>)],
+        index: usize,
+        mark: &Mark<'a>,
+    ) -> Option<&'b Mark<'a>> {
+        Some(
+            &state[index..]
+                .iter()
+                .find(|(_, m)| m.name() == mark.name())?
+                .1,
+        )
+    }
+
+    fn mark_below<'b>(
+        state: &'b mut [(OpId, Mark<'a>)],
+        index: usize,
+        mark: &Mark<'a>,
+    ) -> Option<&'b mut Mark<'a>> {
+        Some(
+            &mut state[0..index]
+                .iter_mut()
+                .filter(|(_, m)| m.data.name == mark.data.name)
+                .last()?
+                .1,
+        )
+    }
+}
+
+#[derive(PartialEq, Debug, Clone)]
+pub struct MarkData {
+    pub name: SmolStr,
+    pub value: ScalarValue,
+}
+
+impl Display for MarkData {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "name={} value={}", self.name, self.value)
+    }
+}
+
+#[derive(PartialEq, Debug, Clone, Copy)]
+pub enum ExpandMark {
+    Left,
+    Right,
+    Both,
+    None,
+}
+
+impl ExpandMark {
+    pub fn from(left: bool, right: bool) -> Self {
+        match (left, right) {
+            (true, true) => Self::Both,
+            (false, true) => Self::Right,
+            (true, false) => Self::Left,
+            (false, false) => Self::None,
+        }
+    }
+    pub fn left(&self) -> bool {
+        matches!(self, Self::Left | Self::Both)
+    }
+    pub fn right(&self) -> bool {
+        matches!(self, Self::Right | Self::Both)
+    }
+}
diff --git a/rust/automerge/src/op_observer.rs b/rust/automerge/src/op_observer.rs
index 5b33c21f..6870c136 100644
--- a/rust/automerge/src/op_observer.rs
+++ b/rust/automerge/src/op_observer.rs
@@ -1,10 +1,17 @@
 use crate::exid::ExId;
+use crate::marks::Mark;
 use crate::Prop;
 use crate::ReadDoc;
 use crate::Value;
 
 mod compose;
+mod patch;
+mod toggle_observer;
+mod vec_observer;
 pub use compose::compose;
+pub use patch::{Patch, PatchAction};
+pub use toggle_observer::ToggleObserver;
+pub use vec_observer::{HasPatches, TextRepresentation, VecOpObserver, VecOpObserver16};
 
 /// An observer of operations applied to the document.
 pub trait OpObserver {
@@ -21,6 +28,7 @@ pub trait OpObserver {
         objid: ExId,
         index: usize,
         tagged_value: (Value<'_>, ExId),
+        conflict: bool,
     );
 
     /// Some text has been spliced into a text object
@@ -111,6 +119,15 @@ pub trait OpObserver {
     /// - `num`: the number of sequential elements deleted
     fn delete_seq<R: ReadDoc>(&mut self, doc: &R, objid: ExId, index: usize, num: usize);
 
+    fn mark<'a, R: ReadDoc, M: Iterator<Item = Mark<'a>>>(
+        &mut self,
+        doc: &'a R,
+        objid: ExId,
+        mark: M,
+    );
+
+    fn unmark<R: ReadDoc>(&mut self, doc: &R, objid: ExId, name: &str, start: usize, end: usize);
+
     /// Whether to call sequence methods or `splice_text` when encountering changes in text
     ///
     /// Returns `false` by default
@@ -146,6 +163,7 @@ impl OpObserver for () {
         _objid: ExId,
         _index: usize,
         _tagged_value: (Value<'_>, ExId),
+        _conflict: bool,
     ) {
     }
 
@@ -180,6 +198,24 @@ impl OpObserver for () {
     ) {
     }
 
+    fn mark<'a, R: ReadDoc, M: Iterator<Item = Mark<'a>>>(
+        &mut self,
+        _doc: &'a R,
+        _objid: ExId,
+        _mark: M,
+    ) {
+    }
+
+    fn unmark<R: ReadDoc>(
+        &mut self,
+        _doc: &R,
+        _objid: ExId,
+        _name: &str,
+        _start: usize,
+        _end: usize,
+    ) {
+    }
+
     fn delete_map<R: ReadDoc>(&mut self, _doc: &R, _objid: ExId, _key: &str) {}
 
     fn delete_seq<R: ReadDoc>(&mut self, _doc: &R, _objid: ExId, _index: usize, _num: usize) {}
@@ -189,204 +225,3 @@ impl BranchableObserver for () {
     fn merge(&mut self, _other: &Self) {}
     fn branch(&self) -> Self {}
 }
-
-/// Capture operations into a [`Vec`] and store them as patches.
-#[derive(Default, Debug, Clone)]
-pub struct VecOpObserver {
-    patches: Vec<Patch>,
-}
-
-impl VecOpObserver {
-    /// Take the current list of patches, leaving the internal list empty and ready for new
-    /// patches.
-    pub fn take_patches(&mut self) -> Vec<Patch> {
-        std::mem::take(&mut self.patches)
-    }
-}
-
-impl OpObserver for VecOpObserver {
-    fn insert<R: ReadDoc>(
-        &mut self,
-        doc: &R,
-        obj: ExId,
-        index: usize,
-        (value, id): (Value<'_>, ExId),
-    ) {
-        if let Ok(p) = doc.parents(&obj) {
-            self.patches.push(Patch::Insert {
-                obj,
-                path: p.path(),
-                index,
-                value: (value.into_owned(), id),
-            });
-        }
-    }
-
-    fn splice_text<R: ReadDoc>(&mut self, doc: &R, obj: ExId, index: usize, value: &str) {
-        if let Ok(p) = doc.parents(&obj) {
-            self.patches.push(Patch::Splice {
-                obj,
-                path: p.path(),
-                index,
-                value: value.to_string(),
-            })
-        }
-    }
-
-    fn put<R: ReadDoc>(
-        &mut self,
-        doc: &R,
-        obj: ExId,
-        prop: Prop,
-        (value, id): (Value<'_>, ExId),
-        conflict: bool,
-    ) {
-        if let Ok(p) = doc.parents(&obj) {
-            self.patches.push(Patch::Put {
-                obj,
-                path: p.path(),
-                prop,
-                value: (value.into_owned(), id),
-                conflict,
-            });
-        }
-    }
-
-    fn expose<R: ReadDoc>(
-        &mut self,
-        doc: &R,
-        obj: ExId,
-        prop: Prop,
-        (value, id): (Value<'_>, ExId),
-        conflict: bool,
-    ) {
-        if let Ok(p) = doc.parents(&obj) {
-            self.patches.push(Patch::Expose {
-                obj,
-                path: p.path(),
-                prop,
-                value: (value.into_owned(), id),
-                conflict,
-            });
-        }
-    }
-
-    fn increment<R: ReadDoc>(&mut self, doc: &R, obj: ExId, prop: Prop, tagged_value: (i64, ExId)) {
-        if let Ok(p) = doc.parents(&obj) {
-            self.patches.push(Patch::Increment {
-                obj,
-                path: p.path(),
-                prop,
-                value: tagged_value,
-            });
-        }
-    }
-
-    fn delete_map<R: ReadDoc>(&mut self, doc: &R, obj: ExId, key: &str) {
-        if let Ok(p) = doc.parents(&obj) {
-            self.patches.push(Patch::Delete {
-                obj,
-                path: p.path(),
-                prop: Prop::Map(key.to_owned()),
-                num: 1,
-            })
-        }
-    }
-
-    fn delete_seq<R: ReadDoc>(&mut self, doc: &R, obj: ExId, index: usize, num: usize) {
-        if let Ok(p) = doc.parents(&obj) {
-            self.patches.push(Patch::Delete {
-                obj,
-                path: p.path(),
-                prop: Prop::Seq(index),
-                num,
-            })
-        }
-    }
-}
-
-impl BranchableObserver for VecOpObserver {
-    fn merge(&mut self, other: &Self) {
-        self.patches.extend_from_slice(other.patches.as_slice())
-    }
-
-    fn branch(&self) -> Self {
-        Self::default()
-    }
-}
-
-/// A notification to the application that something has changed in a document.
-#[derive(Debug, Clone, PartialEq)]
-pub enum Patch {
-    /// Associating a new value with a prop in a map, or an existing list element
-    Put {
-        /// path to the object
-        path: Vec<(ExId, Prop)>,
-        /// The object that was put into.
-        obj: ExId,
-        /// The prop that the new value was put at.
-        prop: Prop,
-        /// The value that was put, and the id of the operation that put it there.
-        value: (Value<'static>, ExId),
-        /// Whether this put conflicts with another.
-        conflict: bool,
-    },
-    /// Exposing (via delete) an old but conflicted value with a prop in a map, or a list element
-    Expose {
-        /// path to the object
-        path: Vec<(ExId, Prop)>,
-        /// The object that was put into.
-        obj: ExId,
-        /// The prop that the new value was put at.
-        prop: Prop,
-        /// The value that was put, and the id of the operation that put it there.
-        value: (Value<'static>, ExId),
-        /// Whether this put conflicts with another.
-        conflict: bool,
-    },
-    /// Inserting a new element into a list
-    Insert {
-        /// path to the object
-        path: Vec<(ExId, Prop)>,
-        /// The object that was inserted into.
-        obj: ExId,
-        /// The index that the new value was inserted at.
-        index: usize,
-        /// The value that was inserted, and the id of the operation that inserted it there.
-        value: (Value<'static>, ExId),
-    },
-    /// Splicing a text object
-    Splice {
-        /// path to the object
-        path: Vec<(ExId, Prop)>,
-        /// The object that was inserted into.
-        obj: ExId,
-        /// The index that the new value was inserted at.
-        index: usize,
-        /// The value that was spliced
-        value: String,
-    },
-    /// Incrementing a counter.
-    Increment {
-        /// path to the object
-        path: Vec<(ExId, Prop)>,
-        /// The object that was incremented in.
-        obj: ExId,
-        /// The prop that was incremented.
-        prop: Prop,
-        /// The amount that the counter was incremented by, and the id of the operation that
-        /// did the increment.
-        value: (i64, ExId),
-    },
-    /// Deleting an element from a list/text
-    Delete {
-        /// path to the object
-        path: Vec<(ExId, Prop)>,
-        /// The object that was deleted from.
-        obj: ExId,
-        /// The prop that was deleted.
-        prop: Prop,
-        /// number of items deleted (for seq)
-        num: usize,
-    },
-}
diff --git a/rust/automerge/src/op_observer/compose.rs b/rust/automerge/src/op_observer/compose.rs
index 92fe3b1e..0dfc6542 100644
--- a/rust/automerge/src/op_observer/compose.rs
+++ b/rust/automerge/src/op_observer/compose.rs
@@ -19,10 +19,11 @@ impl<'a, O1: OpObserver, O2: OpObserver> OpObserver for ComposeObservers<'a, O1,
         objid: crate::ObjId,
         index: usize,
         tagged_value: (crate::Value<'_>, crate::ObjId),
+        conflict: bool,
     ) {
         self.obs1
-            .insert(doc, objid.clone(), index, tagged_value.clone());
-        self.obs2.insert(doc, objid, index, tagged_value);
+            .insert(doc, objid.clone(), index, tagged_value.clone(), conflict);
+        self.obs2.insert(doc, objid, index, tagged_value, conflict);
     }
 
     fn splice_text<R: crate::ReadDoc>(
@@ -84,6 +85,30 @@ impl<'a, O1: OpObserver, O2: OpObserver> OpObserver for ComposeObservers<'a, O1,
         self.obs2.increment(doc, objid, prop, tagged_value);
     }
 
+    fn mark<'b, R: crate::ReadDoc, M: Iterator<Item = crate::marks::Mark<'b>>>(
+        &mut self,
+        doc: &'b R,
+        objid: crate::ObjId,
+        mark: M,
+    ) {
+        let marks: Vec<_> = mark.collect();
+        self.obs1
+            .mark(doc, objid.clone(), marks.clone().into_iter());
+        self.obs2.mark(doc, objid, marks.into_iter());
+    }
+
+    fn unmark<R: crate::ReadDoc>(
+        &mut self,
+        doc: &R,
+        objid: crate::ObjId,
+        name: &str,
+        start: usize,
+        end: usize,
+    ) {
+        self.obs1.unmark(doc, objid.clone(), name, start, end);
+        self.obs2.unmark(doc, objid, name, start, end);
+    }
+
     fn delete_map<R: crate::ReadDoc>(&mut self, doc: &R, objid: crate::ObjId, key: &str) {
         self.obs1.delete_map(doc, objid.clone(), key);
         self.obs2.delete_map(doc, objid, key);
diff --git a/rust/automerge/src/op_observer/patch.rs b/rust/automerge/src/op_observer/patch.rs
new file mode 100644
index 00000000..717f680b
--- /dev/null
+++ b/rust/automerge/src/op_observer/patch.rs
@@ -0,0 +1,57 @@
+#![allow(dead_code)]
+
+use crate::{marks::Mark, ObjId, Prop, Value};
+use core::fmt::Debug;
+
+use crate::sequence_tree::SequenceTree;
+
+#[derive(Debug, Clone, PartialEq)]
+pub struct Patch<T: PartialEq + Clone + Debug> {
+    pub obj: ObjId,
+    pub path: Vec<(ObjId, Prop)>,
+    pub action: PatchAction<T>,
+}
+
+#[derive(Debug, Clone, PartialEq)]
+pub enum PatchAction<T: PartialEq + Clone + Debug> {
+    PutMap {
+        key: String,
+        value: (Value<'static>, ObjId),
+        expose: bool,
+        conflict: bool,
+    },
+    PutSeq {
+        index: usize,
+        value: (Value<'static>, ObjId),
+        expose: bool,
+        conflict: bool,
+    },
+    Insert {
+        index: usize,
+        values: SequenceTree<(Value<'static>, ObjId)>,
+        conflict: bool,
+    },
+    SpliceText {
+        index: usize,
+        value: SequenceTree<T>,
+    },
+    Increment {
+        prop: Prop,
+        value: i64,
+    },
+    DeleteMap {
+        key: String,
+    },
+    DeleteSeq {
+        index: usize,
+        length: usize,
+    },
+    Mark {
+        marks: Vec<Mark<'static>>,
+    },
+    Unmark {
+        name: String,
+        start: usize,
+        end: usize,
+    },
+}
diff --git a/rust/automerge/src/op_observer/toggle_observer.rs b/rust/automerge/src/op_observer/toggle_observer.rs
new file mode 100644
index 00000000..6afcee9f
--- /dev/null
+++ b/rust/automerge/src/op_observer/toggle_observer.rs
@@ -0,0 +1,178 @@
+#![allow(dead_code)]
+
+use crate::ChangeHash;
+use core::fmt::Debug;
+
+use crate::{marks::Mark, ObjId, OpObserver, Prop, ReadDoc, Value};
+
+use crate::op_observer::BranchableObserver;
+use crate::op_observer::{HasPatches, TextRepresentation};
+
+#[derive(Debug, Clone)]
+pub struct ToggleObserver<T> {
+    enabled: bool,
+    last_heads: Option<Vec<ChangeHash>>,
+    observer: T,
+}
+
+impl<T: Default> Default for ToggleObserver<T> {
+    fn default() -> Self {
+        Self {
+            enabled: false,
+            last_heads: None,
+            observer: T::default(),
+        }
+    }
+}
+
+impl<T: HasPatches> ToggleObserver<T> {
+    pub fn new(observer: T) -> Self {
+        ToggleObserver {
+            enabled: false,
+            last_heads: None,
+            observer,
+        }
+    }
+
+    pub fn take_patches(&mut self, heads: Vec<ChangeHash>) -> (T::Patches, Vec<ChangeHash>) {
+        let old_heads = self.last_heads.replace(heads).unwrap_or_default();
+        let patches = self.observer.take_patches();
+        (patches, old_heads)
+    }
+
+    pub fn with_text_rep(mut self, text_rep: TextRepresentation) -> Self {
+        self.observer = self.observer.with_text_rep(text_rep);
+        self
+    }
+
+    pub fn set_text_rep(&mut self, text_rep: TextRepresentation) {
+        self.observer.set_text_rep(text_rep)
+    }
+
+    pub fn enable(&mut self, enable: bool, heads: Vec<ChangeHash>) -> bool {
+        if self.enabled && !enable {
+            self.observer.take_patches();
+            self.last_heads = Some(heads);
+        }
+        let old_enabled = self.enabled;
+        self.enabled = enable;
+        old_enabled
+    }
+
+    fn get_path<R: ReadDoc>(&mut self, doc: &R, obj: &ObjId) -> Option<Vec<(ObjId, Prop)>> {
+        match doc.parents(obj) {
+            Ok(parents) => parents.visible_path(),
+            Err(e) => {
+                log!("error generating patch : {:?}", e);
+                None
+            }
+        }
+    }
+}
+
+impl<T: OpObserver + HasPatches> OpObserver for ToggleObserver<T> {
+    fn insert<R: ReadDoc>(
+        &mut self,
+        doc: &R,
+        obj: ObjId,
+        index: usize,
+        tagged_value: (Value<'_>, ObjId),
+        conflict: bool,
+    ) {
+        if self.enabled {
+            self.observer
+                .insert(doc, obj, index, tagged_value, conflict)
+        }
+    }
+
+    fn splice_text<R: ReadDoc>(&mut self, doc: &R, obj: ObjId, index: usize, value: &str) {
+        if self.enabled {
+            self.observer.splice_text(doc, obj, index, value)
+        }
+    }
+
+    fn delete_seq<R: ReadDoc>(&mut self, doc: &R, obj: ObjId, index: usize, length: usize) {
+        if self.enabled {
+            self.observer.delete_seq(doc, obj, index, length)
+        }
+    }
+
+    fn delete_map<R: ReadDoc>(&mut self, doc: &R, obj: ObjId, key: &str) {
+        if self.enabled {
+            self.observer.delete_map(doc, obj, key)
+        }
+    }
+
+    fn put<R: ReadDoc>(
+        &mut self,
+        doc: &R,
+        obj: ObjId,
+        prop: Prop,
+        tagged_value: (Value<'_>, ObjId),
+        conflict: bool,
+    ) {
+        if self.enabled {
+            self.observer.put(doc, obj, prop, tagged_value, conflict)
+        }
+    }
+
+    fn expose<R: ReadDoc>(
+        &mut self,
+        doc: &R,
+        obj: ObjId,
+        prop: Prop,
+        tagged_value: (Value<'_>, ObjId),
+        conflict: bool,
+    ) {
+        if self.enabled {
+            self.observer.expose(doc, obj, prop, tagged_value, conflict)
+        }
+    }
+
+    fn increment<R: ReadDoc>(
+        &mut self,
+        doc: &R,
+        obj: ObjId,
+        prop: Prop,
+        tagged_value: (i64, ObjId),
+    ) {
+        if self.enabled {
+            self.observer.increment(doc, obj, prop, tagged_value)
+        }
+    }
+
+    fn mark<'a, R: ReadDoc, M: Iterator<Item = Mark<'a>>>(
+        &mut self,
+        doc: &'a R,
+        obj: ObjId,
+        mark: M,
+    ) {
+        if self.enabled {
+            self.observer.mark(doc, obj, mark)
+        }
+    }
+
+    fn unmark<R: ReadDoc>(&mut self, doc: &R, obj: ObjId, name: &str, start: usize, end: usize) {
+        if self.enabled {
+            self.observer.unmark(doc, obj, name, start, end)
+        }
+    }
+
+    fn text_as_seq(&self) -> bool {
+        self.observer.get_text_rep() == TextRepresentation::Array
+    }
+}
+
+impl<T: BranchableObserver> BranchableObserver for ToggleObserver<T> {
+    fn merge(&mut self, other: &Self) {
+        self.observer.merge(&other.observer)
+    }
+
+    fn branch(&self) -> Self {
+        ToggleObserver {
+            observer: self.observer.branch(),
+            last_heads: None,
+            enabled: self.enabled,
+        }
+    }
+}
diff --git a/rust/automerge/src/op_observer/vec_observer.rs b/rust/automerge/src/op_observer/vec_observer.rs
new file mode 100644
index 00000000..9887699a
--- /dev/null
+++ b/rust/automerge/src/op_observer/vec_observer.rs
@@ -0,0 +1,561 @@
+#![allow(dead_code)]
+
+use core::fmt::Debug;
+
+use crate::{marks::Mark, ObjId, OpObserver, Prop, ReadDoc, ScalarValue, Value};
+
+use crate::sequence_tree::SequenceTree;
+
+use crate::op_observer::BranchableObserver;
+use crate::op_observer::{Patch, PatchAction};
+
+#[derive(Debug, Copy, Clone, PartialEq)]
+pub enum TextRepresentation {
+    Array,
+    String,
+}
+
+impl TextRepresentation {
+    pub fn is_array(&self) -> bool {
+        matches!(self, TextRepresentation::Array)
+    }
+
+    pub fn is_string(&self) -> bool {
+        matches!(self, TextRepresentation::String)
+    }
+}
+
+impl std::default::Default for TextRepresentation {
+    fn default() -> Self {
+        TextRepresentation::Array
+    }
+}
+
+pub(crate) trait TextIndex {
+    type Item: Debug + PartialEq + Clone;
+    type Iter<'a>: Iterator<Item = Self::Item>;
+
+    fn chars(text: &str) -> Self::Iter<'_>;
+}
+
+#[derive(Debug, Clone, Default)]
+struct VecOpObserverInner<T: TextIndex> {
+    pub(crate) patches: Vec<Patch<T::Item>>,
+    pub(crate) text_rep: TextRepresentation,
+}
+
+#[derive(Debug, Clone, Default)]
+pub struct VecOpObserver(VecOpObserverInner<Utf8TextIndex>);
+
+#[derive(Debug, Clone, Default)]
+pub struct VecOpObserver16(VecOpObserverInner<Utf16TextIndex>);
+
+#[derive(Debug, Clone, Default)]
+pub(crate) struct Utf16TextIndex;
+
+#[derive(Debug, Clone, Default)]
+pub(crate) struct Utf8TextIndex;
+
+impl TextIndex for Utf8TextIndex {
+    type Item = char;
+    type Iter<'a> = std::str::Chars<'a>;
+
+    fn chars(text: &str) -> Self::Iter<'_> {
+        text.chars()
+    }
+}
+
+impl TextIndex for Utf16TextIndex {
+    type Item = u16;
+    type Iter<'a> = std::str::EncodeUtf16<'a>;
+
+    fn chars(text: &str) -> Self::Iter<'_> {
+        text.encode_utf16()
+    }
+}
+
+pub trait HasPatches {
+    type Patches;
+
+    fn take_patches(&mut self) -> Self::Patches;
+    fn with_text_rep(self, text_rep: TextRepresentation) -> Self;
+    fn set_text_rep(&mut self, text_rep: TextRepresentation);
+    fn get_text_rep(&self) -> TextRepresentation;
+}
+
+impl HasPatches for VecOpObserver {
+    type Patches = Vec<Patch<char>>;
+
+    fn take_patches(&mut self) -> Self::Patches {
+        std::mem::take(&mut self.0.patches)
+    }
+
+    fn with_text_rep(mut self, text_rep: TextRepresentation) -> Self {
+        self.0.text_rep = text_rep;
+        self
+    }
+
+    fn set_text_rep(&mut self, text_rep: TextRepresentation) {
+        self.0.text_rep = text_rep;
+    }
+
+    fn get_text_rep(&self) -> TextRepresentation {
+        self.0.text_rep
+    }
+}
+
+impl HasPatches for VecOpObserver16 {
+    type Patches = Vec<Patch<u16>>;
+
+    fn take_patches(&mut self) -> Self::Patches {
+        std::mem::take(&mut self.0.patches)
+    }
+
+    fn with_text_rep(mut self, text_rep: TextRepresentation) -> Self {
+        self.0.text_rep = text_rep;
+        self
+    }
+
+    fn set_text_rep(&mut self, text_rep: TextRepresentation) {
+        self.0.text_rep = text_rep;
+    }
+
+    fn get_text_rep(&self) -> TextRepresentation {
+        self.0.text_rep
+    }
+}
+
+impl<T: TextIndex> VecOpObserverInner<T> {
+    fn get_path<R: ReadDoc>(&mut self, doc: &R, obj: &ObjId) -> Option<Vec<(ObjId, Prop)>> {
+        match doc.parents(obj) {
+            Ok(parents) => parents.visible_path(),
+            Err(e) => {
+                log!("error generating patch : {:?}", e);
+                None
+            }
+        }
+    }
+
+    fn maybe_append(&mut self, obj: &ObjId) -> Option<&mut PatchAction<T::Item>> {
+        match self.patches.last_mut() {
+            Some(Patch {
+                obj: tail_obj,
+                action,
+                ..
+            }) if obj == tail_obj => Some(action),
+            _ => None,
+        }
+    }
+}
+
+impl<T: TextIndex> OpObserver for VecOpObserverInner<T> {
+    fn insert<R: ReadDoc>(
+        &mut self,
+        doc: &R,
+        obj: ObjId,
+        index: usize,
+        tagged_value: (Value<'_>, ObjId),
+        conflict: bool,
+    ) {
+        let value = (tagged_value.0.to_owned(), tagged_value.1);
+        if let Some(PatchAction::Insert {
+            index: tail_index,
+            values,
+            ..
+        }) = self.maybe_append(&obj)
+        {
+            let range = *tail_index..=*tail_index + values.len();
+            if range.contains(&index) {
+                values.insert(index - *tail_index, value);
+                return;
+            }
+        }
+        if let Some(path) = self.get_path(doc, &obj) {
+            let mut values = SequenceTree::new();
+            values.push(value);
+            let action = PatchAction::Insert {
+                index,
+                values,
+                conflict,
+            };
+            self.patches.push(Patch { obj, path, action });
+        }
+    }
+
+    fn splice_text<R: ReadDoc>(&mut self, doc: &R, obj: ObjId, index: usize, value: &str) {
+        if self.text_rep == TextRepresentation::Array {
+            for (offset, c) in value.chars().map(ScalarValue::from).enumerate() {
+                let value = (c.into(), ObjId::Root);
+                self.insert(doc, obj.clone(), index + offset, value, false);
+            }
+            return;
+        }
+        if let Some(PatchAction::SpliceText {
+            index: tail_index,
+            value: prev_value,
+            ..
+        }) = self.maybe_append(&obj)
+        {
+            let range = *tail_index..=*tail_index + prev_value.len();
+            if range.contains(&index) {
+                let i = index - *tail_index;
+                for (n, ch) in T::chars(value).enumerate() {
+                    prev_value.insert(i + n, ch)
+                }
+                return;
+            }
+        }
+        if let Some(path) = self.get_path(doc, &obj) {
+            let mut v = SequenceTree::new();
+            for ch in T::chars(value) {
+                v.push(ch)
+            }
+            let action = PatchAction::SpliceText { index, value: v };
+            self.patches.push(Patch { obj, path, action });
+        }
+    }
+
+    fn delete_seq<R: ReadDoc>(&mut self, doc: &R, obj: ObjId, index: usize, length: usize) {
+        match self.maybe_append(&obj) {
+            Some(PatchAction::SpliceText {
+                index: tail_index,
+                value,
+                ..
+            }) => {
+                let range = *tail_index..*tail_index + value.len();
+                if range.contains(&index) && range.contains(&(index + length - 1)) {
+                    for _ in 0..length {
+                        value.remove(index - *tail_index);
+                    }
+                    return;
+                }
+            }
+            Some(PatchAction::Insert {
+                index: tail_index,
+                values,
+                ..
+            }) => {
+                let range = *tail_index..*tail_index + values.len();
+                if range.contains(&index) && range.contains(&(index + length - 1)) {
+                    for _ in 0..length {
+                        values.remove(index - *tail_index);
+                    }
+                    return;
+                }
+            }
+            Some(PatchAction::DeleteSeq {
+                index: tail_index,
+                length: tail_length,
+                ..
+            }) => {
+                if index == *tail_index {
+                    *tail_length += length;
+                    return;
+                }
+            }
+            _ => {}
+        }
+        if let Some(path) = self.get_path(doc, &obj) {
+            let action = PatchAction::DeleteSeq { index, length };
+            self.patches.push(Patch { obj, path, action })
+        }
+    }
+
+    fn delete_map<R: ReadDoc>(&mut self, doc: &R, obj: ObjId, key: &str) {
+        if let Some(path) = self.get_path(doc, &obj) {
+            let action = PatchAction::DeleteMap {
+                key: key.to_owned(),
+            };
+            self.patches.push(Patch { obj, path, action })
+        }
+    }
+
+    fn put<R: ReadDoc>(
+        &mut self,
+        doc: &R,
+        obj: ObjId,
+        prop: Prop,
+        tagged_value: (Value<'_>, ObjId),
+        conflict: bool,
+    ) {
+        let expose = false;
+        if let Some(path) = self.get_path(doc, &obj) {
+            let value = (tagged_value.0.to_owned(), tagged_value.1);
+            let action = match prop {
+                Prop::Map(key) => PatchAction::PutMap {
+                    key,
+                    value,
+                    expose,
+                    conflict,
+                },
+                Prop::Seq(index) => PatchAction::PutSeq {
+                    index,
+                    value,
+                    expose,
+                    conflict,
+                },
+            };
+            self.patches.push(Patch { obj, path, action })
+        }
+    }
+
+    fn expose<R: ReadDoc>(
+        &mut self,
+        doc: &R,
+        obj: ObjId,
+        prop: Prop,
+        tagged_value: (Value<'_>, ObjId),
+        conflict: bool,
+    ) {
+        let expose = true;
+        if let Some(path) = self.get_path(doc, &obj) {
+            let value = (tagged_value.0.to_owned(), tagged_value.1);
+            let action = match prop {
+                Prop::Map(key) => PatchAction::PutMap {
+                    key,
+                    value,
+                    expose,
+                    conflict,
+                },
+                Prop::Seq(index) => PatchAction::PutSeq {
+                    index,
+                    value,
+                    expose,
+                    conflict,
+                },
+            };
+            self.patches.push(Patch { obj, path, action })
+        }
+    }
+
+    fn increment<R: ReadDoc>(
+        &mut self,
+        doc: &R,
+        obj: ObjId,
+        prop: Prop,
+        tagged_value: (i64, ObjId),
+    ) {
+        if let Some(path) = self.get_path(doc, &obj) {
+            let value = tagged_value.0;
+            let action = PatchAction::Increment { prop, value };
+            self.patches.push(Patch { obj, path, action })
+        }
+    }
+
+    fn mark<'a, R: ReadDoc, M: Iterator<Item = Mark<'a>>>(
+        &mut self,
+        doc: &'a R,
+        obj: ObjId,
+        mark: M,
+    ) {
+        if let Some(PatchAction::Mark { marks, .. }) = self.maybe_append(&obj) {
+            for m in mark {
+                marks.push(m.into_owned())
+            }
+            return;
+        }
+        if let Some(path) = self.get_path(doc, &obj) {
+            let marks: Vec<_> = mark.map(|m| m.into_owned()).collect();
+            if !marks.is_empty() {
+                let action = PatchAction::Mark { marks };
+                self.patches.push(Patch { obj, path, action });
+            }
+        }
+    }
+
+    fn unmark<R: ReadDoc>(&mut self, doc: &R, obj: ObjId, name: &str, start: usize, end: usize) {
+        if let Some(path) = self.get_path(doc, &obj) {
+            let action = PatchAction::Unmark {
+                name: name.to_string(),
+                start,
+                end,
+            };
+            self.patches.push(Patch { obj, path, action });
+        }
+    }
+
+    fn text_as_seq(&self) -> bool {
+        self.text_rep == TextRepresentation::Array
+    }
+}
+
+impl<T: TextIndex> BranchableObserver for VecOpObserverInner<T> {
+    fn merge(&mut self, other: &Self) {
+        self.patches.extend_from_slice(other.patches.as_slice())
+    }
+
+    fn branch(&self) -> Self {
+        VecOpObserverInner {
+            patches: vec![],
+            text_rep: self.text_rep,
+        }
+    }
+}
+
+impl OpObserver for VecOpObserver {
+    fn insert<R: ReadDoc>(
+        &mut self,
+        doc: &R,
+        obj: ObjId,
+        index: usize,
+        tagged_value: (Value<'_>, ObjId),
+        conflict: bool,
+    ) {
+        self.0.insert(doc, obj, index, tagged_value, conflict)
+    }
+
+    fn splice_text<R: ReadDoc>(&mut self, doc: &R, obj: ObjId, index: usize, value: &str) {
+        self.0.splice_text(doc, obj, index, value)
+    }
+
+    fn delete_seq<R: ReadDoc>(&mut self, doc: &R, obj: ObjId, index: usize, length: usize) {
+        self.0.delete_seq(doc, obj, index, length)
+    }
+
+    fn delete_map<R: ReadDoc>(&mut self, doc: &R, obj: ObjId, key: &str) {
+        self.0.delete_map(doc, obj, key)
+    }
+
+    fn put<R: ReadDoc>(
+        &mut self,
+        doc: &R,
+        obj: ObjId,
+        prop: Prop,
+        tagged_value: (Value<'_>, ObjId),
+        conflict: bool,
+    ) {
+        self.0.put(doc, obj, prop, tagged_value, conflict)
+    }
+
+    fn expose<R: ReadDoc>(
+        &mut self,
+        doc: &R,
+        obj: ObjId,
+        prop: Prop,
+        tagged_value: (Value<'_>, ObjId),
+        conflict: bool,
+    ) {
+        self.0.expose(doc, obj, prop, tagged_value, conflict)
+    }
+
+    fn increment<R: ReadDoc>(
+        &mut self,
+        doc: &R,
+        obj: ObjId,
+        prop: Prop,
+        tagged_value: (i64, ObjId),
+    ) {
+        self.0.increment(doc, obj, prop, tagged_value)
+    }
+
+    fn mark<'a, R: ReadDoc, M: Iterator<Item = Mark<'a>>>(
+        &mut self,
+        doc: &'a R,
+        obj: ObjId,
+        mark: M,
+    ) {
+        self.0.mark(doc, obj, mark)
+    }
+
+    fn unmark<R: ReadDoc>(&mut self, doc: &R, obj: ObjId, name: &str, start: usize, end: usize) {
+        self.0.unmark(doc, obj, name, start, end)
+    }
+
+    fn text_as_seq(&self) -> bool {
+        self.0.text_as_seq()
+    }
+}
+
+impl OpObserver for VecOpObserver16 {
+    fn insert<R: ReadDoc>(
+        &mut self,
+        doc: &R,
+        obj: ObjId,
+        index: usize,
+        tagged_value: (Value<'_>, ObjId),
+        conflict: bool,
+    ) {
+        self.0.insert(doc, obj, index, tagged_value, conflict)
+    }
+
+    fn splice_text<R: ReadDoc>(&mut self, doc: &R, obj: ObjId, index: usize, value: &str) {
+        self.0.splice_text(doc, obj, index, value)
+    }
+
+    fn delete_seq<R: ReadDoc>(&mut self, doc: &R, obj: ObjId, index: usize, length: usize) {
+        self.0.delete_seq(doc, obj, index, length)
+    }
+
+    fn delete_map<R: ReadDoc>(&mut self, doc: &R, obj: ObjId, key: &str) {
+        self.0.delete_map(doc, obj, key)
+    }
+
+    fn put<R: ReadDoc>(
+        &mut self,
+        doc: &R,
+        obj: ObjId,
+        prop: Prop,
+        tagged_value: (Value<'_>, ObjId),
+        conflict: bool,
+    ) {
+        self.0.put(doc, obj, prop, tagged_value, conflict)
+    }
+
+    fn expose<R: ReadDoc>(
+        &mut self,
+        doc: &R,
+        obj: ObjId,
+        prop: Prop,
+        tagged_value: (Value<'_>, ObjId),
+        conflict: bool,
+    ) {
+        self.0.expose(doc, obj, prop, tagged_value, conflict)
+    }
+
+    fn increment<R: ReadDoc>(
+        &mut self,
+        doc: &R,
+        obj: ObjId,
+        prop: Prop,
+        tagged_value: (i64, ObjId),
+    ) {
+        self.0.increment(doc, obj, prop, tagged_value)
+    }
+
+    fn mark<'a, R: ReadDoc, M: Iterator<Item = Mark<'a>>>(
+        &mut self,
+        doc: &'a R,
+        obj: ObjId,
+        mark: M,
+    ) {
+        self.0.mark(doc, obj, mark)
+    }
+
+    fn unmark<R: ReadDoc>(&mut self, doc: &R, obj: ObjId, name: &str, start: usize, end: usize) {
+        self.0.unmark(doc, obj, name, start, end)
+    }
+
+    fn text_as_seq(&self) -> bool {
+        self.0.text_as_seq()
+    }
+}
+
+impl BranchableObserver for VecOpObserver {
+    fn merge(&mut self, other: &Self) {
+        self.0.merge(&other.0)
+    }
+
+    fn branch(&self) -> Self {
+        VecOpObserver(self.0.branch())
+    }
+}
+
+impl BranchableObserver for VecOpObserver16 {
+    fn merge(&mut self, other: &Self) {
+        self.0.merge(&other.0)
+    }
+
+    fn branch(&self) -> Self {
+        VecOpObserver16(self.0.branch())
+    }
+}
diff --git a/rust/automerge/src/op_set.rs b/rust/automerge/src/op_set.rs
index aab8ce74..cd697abd 100644
--- a/rust/automerge/src/op_set.rs
+++ b/rust/automerge/src/op_set.rs
@@ -78,6 +78,10 @@ impl OpSetInternal {
         }
     }
 
+    pub(crate) fn iter_ops(&self, obj: &ObjId) -> impl Iterator<Item = &Op> {
+        self.trees.get(obj).map(|o| o.iter()).into_iter().flatten()
+    }
+
     pub(crate) fn parents(&self, obj: ObjId) -> Parents<'_> {
         Parents { obj, ops: self }
     }
diff --git a/rust/automerge/src/op_tree.rs b/rust/automerge/src/op_tree.rs
index 7de00dc3..60bd2a6e 100644
--- a/rust/automerge/src/op_tree.rs
+++ b/rust/automerge/src/op_tree.rs
@@ -319,8 +319,7 @@ struct CounterData {
 
 #[cfg(test)]
 mod tests {
-    use crate::legacy as amp;
-    use crate::types::{Op, OpId};
+    use crate::types::{Op, OpId, OpType};
 
     use super::*;
 
@@ -328,7 +327,7 @@ mod tests {
         let zero = OpId::new(0, 0);
         Op {
             id: zero,
-            action: amp::OpType::Put(0.into()),
+            action: OpType::Put(0.into()),
             key: zero.into(),
             succ: Default::default(),
             pred: Default::default(),
diff --git a/rust/automerge/src/query.rs b/rust/automerge/src/query.rs
index 640ecf8d..c76c3a0e 100644
--- a/rust/automerge/src/query.rs
+++ b/rust/automerge/src/query.rs
@@ -25,6 +25,7 @@ mod opid;
 mod opid_vis;
 mod prop;
 mod prop_at;
+mod seek_mark;
 mod seek_op;
 mod seek_op_with_patch;
 
@@ -46,6 +47,7 @@ pub(crate) use opid::OpIdSearch;
 pub(crate) use opid_vis::OpIdVisSearch;
 pub(crate) use prop::Prop;
 pub(crate) use prop_at::PropAt;
+pub(crate) use seek_mark::SeekMark;
 pub(crate) use seek_op::SeekOp;
 pub(crate) use seek_op_with_patch::SeekOpWithPatch;
 
@@ -287,7 +289,7 @@ pub(crate) struct VisWindow {
 }
 
 impl VisWindow {
-    fn visible_at(&mut self, op: &Op, pos: usize, clock: &Clock) -> bool {
+    pub(crate) fn visible_at(&mut self, op: &Op, pos: usize, clock: &Clock) -> bool {
         if !clock.covers(&op.id) {
             return false;
         }
diff --git a/rust/automerge/src/query/insert.rs b/rust/automerge/src/query/insert.rs
index 0dc0e98d..8e86d619 100644
--- a/rust/automerge/src/query/insert.rs
+++ b/rust/automerge/src/query/insert.rs
@@ -110,6 +110,12 @@ impl<'a> TreeQuery<'a> for InsertNth {
             self.last_seen = None;
             self.last_insert = element.elemid();
         }
+        /*-------------------*/
+        if self.valid.is_some() && element.valid_mark_anchor() {
+            self.last_valid_insert = Some(element.elemid_or_key());
+            self.valid = None;
+        }
+        /*-------------------*/
         if self.last_seen.is_none() && element.visible() {
             if self.seen >= self.target {
                 return QueryResult::Finish;
diff --git a/rust/automerge/src/query/seek_mark.rs b/rust/automerge/src/query/seek_mark.rs
new file mode 100644
index 00000000..c5834ab8
--- /dev/null
+++ b/rust/automerge/src/query/seek_mark.rs
@@ -0,0 +1,124 @@
+use crate::marks::Mark;
+use crate::op_tree::OpSetMetadata;
+use crate::query::{QueryResult, TreeQuery};
+use crate::types::{Key, ListEncoding, Op, OpId, OpType};
+use std::cmp::Ordering;
+use std::collections::HashMap;
+use std::fmt::Debug;
+
+#[derive(Debug, Clone, PartialEq)]
+pub(crate) struct SeekMark<'a> {
+    /// the mark we are looking for
+    id: OpId,
+    end: usize,
+    encoding: ListEncoding,
+    found: bool,
+    mark_name: smol_str::SmolStr,
+    next_mark: Option<Mark<'a>>,
+    pos: usize,
+    seen: usize,
+    last_seen: Option<Key>,
+    super_marks: HashMap<OpId, smol_str::SmolStr>,
+    pub(crate) marks: Vec<Mark<'a>>,
+}
+
+impl<'a> SeekMark<'a> {
+    pub(crate) fn new(id: OpId, end: usize, encoding: ListEncoding) -> Self {
+        SeekMark {
+            id,
+            encoding,
+            end,
+            found: false,
+            next_mark: None,
+            mark_name: "".into(),
+            pos: 0,
+            seen: 0,
+            last_seen: None,
+            super_marks: Default::default(),
+            marks: Default::default(),
+        }
+    }
+
+    fn count_visible(&mut self, e: &Op) {
+        if e.insert {
+            self.last_seen = None
+        }
+        if e.visible() && self.last_seen.is_none() {
+            self.seen += e.width(self.encoding);
+            self.last_seen = Some(e.elemid_or_key())
+        }
+    }
+}
+
+impl<'a> TreeQuery<'a> for SeekMark<'a> {
+    fn query_element_with_metadata(&mut self, op: &'a Op, m: &OpSetMetadata) -> QueryResult {
+        match &op.action {
+            OpType::MarkBegin(_, data) if op.id == self.id => {
+                if !op.succ.is_empty() {
+                    return QueryResult::Finish;
+                }
+                self.found = true;
+                self.mark_name = data.name.clone();
+                // retain the name and the value
+                self.next_mark = Some(Mark::from_data(self.seen, self.seen, data));
+                // change id to the end id
+                self.id = self.id.next();
+                // remove all marks that dont match
+                self.super_marks.retain(|_, v| v == &data.name);
+            }
+            OpType::MarkBegin(_, mark) => {
+                if m.lamport_cmp(op.id, self.id) == Ordering::Greater {
+                    if let Some(next_mark) = &mut self.next_mark {
+                        // gather marks of the same type that supersede us
+                        if mark.name == self.mark_name {
+                            self.super_marks.insert(op.id.next(), mark.name.clone());
+                            if self.super_marks.len() == 1 {
+                                // complete a mark
+                                next_mark.end = self.seen;
+                                self.marks.push(next_mark.clone());
+                            }
+                        }
+                    } else {
+                        // gather all marks until we know what our mark's name is
+                        self.super_marks.insert(op.id.next(), mark.name.clone());
+                    }
+                }
+            }
+            OpType::MarkEnd(_) if self.end == self.pos => {
+                if self.super_marks.is_empty() {
+                    // complete a mark
+                    if let Some(next_mark) = &mut self.next_mark {
+                        next_mark.end = self.seen;
+                        self.marks.push(next_mark.clone());
+                    }
+                }
+                return QueryResult::Finish;
+            }
+            OpType::MarkEnd(_) if self.super_marks.contains_key(&op.id) => {
+                self.super_marks.remove(&op.id);
+                if let Some(next_mark) = &mut self.next_mark {
+                    if self.super_marks.is_empty() {
+                        // begin a new mark
+                        next_mark.start = self.seen;
+                    }
+                }
+            }
+            _ => {}
+        }
+        // the end op hasn't been inserted yet so we need to work off the position
+        if self.end == self.pos {
+            if self.super_marks.is_empty() {
+                // complete a mark
+                if let Some(next_mark) = &mut self.next_mark {
+                    next_mark.end = self.seen;
+                    self.marks.push(next_mark.clone());
+                }
+            }
+            return QueryResult::Finish;
+        }
+
+        self.pos += 1;
+        self.count_visible(op);
+        QueryResult::Next
+    }
+}
diff --git a/rust/automerge/src/read.rs b/rust/automerge/src/read.rs
index 6d479718..9b3a6a77 100644
--- a/rust/automerge/src/read.rs
+++ b/rust/automerge/src/read.rs
@@ -1,7 +1,7 @@
 use crate::{
     error::AutomergeError, exid::ExId, keys::Keys, keys_at::KeysAt, list_range::ListRange,
-    list_range_at::ListRangeAt, map_range::MapRange, map_range_at::MapRangeAt, parents::Parents,
-    values::Values, Change, ChangeHash, ObjType, Prop, Value,
+    list_range_at::ListRangeAt, map_range::MapRange, map_range_at::MapRangeAt, marks::Mark,
+    parents::Parents, values::Values, Change, ChangeHash, ObjType, Prop, Value,
 };
 
 use std::ops::RangeBounds;
@@ -129,6 +129,16 @@ pub trait ReadDoc {
     /// Get the type of this object, if it is an object.
     fn object_type<O: AsRef<ExId>>(&self, obj: O) -> Result<ObjType, AutomergeError>;
 
+    /// Get all marks on a current sequence
+    fn marks<O: AsRef<ExId>>(&self, obj: O) -> Result<Vec<Mark<'_>>, AutomergeError>;
+
+    /// Get all marks on a sequence at a given heads
+    fn marks_at<O: AsRef<ExId>>(
+        &self,
+        obj: O,
+        heads: &[ChangeHash],
+    ) -> Result<Vec<Mark<'_>>, AutomergeError>;
+
     /// Get the string represented by the given text object.
     fn text<O: AsRef<ExId>>(&self, obj: O) -> Result<String, AutomergeError>;
 
diff --git a/rust/automerge/src/sequence_tree.rs b/rust/automerge/src/sequence_tree.rs
new file mode 100644
index 00000000..920eb13f
--- /dev/null
+++ b/rust/automerge/src/sequence_tree.rs
@@ -0,0 +1,617 @@
+use std::{
+    cmp::{min, Ordering},
+    fmt::Debug,
+    mem,
+};
+
+pub(crate) const B: usize = 16;
+pub(crate) type SequenceTree<T> = SequenceTreeInternal<T>;
+
+#[derive(Clone, Debug)]
+pub struct SequenceTreeInternal<T> {
+    root_node: Option<SequenceTreeNode<T>>,
+}
+
+#[derive(Clone, Debug, PartialEq)]
+struct SequenceTreeNode<T> {
+    elements: Vec<T>,
+    children: Vec<SequenceTreeNode<T>>,
+    length: usize,
+}
+
+impl<T> SequenceTreeInternal<T>
+where
+    T: Clone + Debug,
+{
+    /// Construct a new, empty, sequence.
+    pub fn new() -> Self {
+        Self { root_node: None }
+    }
+
+    /// Get the length of the sequence.
+    pub fn len(&self) -> usize {
+        self.root_node.as_ref().map_or(0, |n| n.len())
+    }
+
+    /// Create an iterator through the sequence.
+    pub fn iter(&self) -> Iter<'_, T> {
+        Iter {
+            inner: self,
+            index: 0,
+        }
+    }
+
+    /// Insert the `element` into the sequence at `index`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `index > len`.
+    pub fn insert(&mut self, index: usize, element: T) {
+        let old_len = self.len();
+        if let Some(root) = self.root_node.as_mut() {
+            #[cfg(debug_assertions)]
+            root.check();
+
+            if root.is_full() {
+                let original_len = root.len();
+                let new_root = SequenceTreeNode::new();
+
+                // move new_root to root position
+                let old_root = mem::replace(root, new_root);
+
+                root.length += old_root.len();
+                root.children.push(old_root);
+                root.split_child(0);
+
+                assert_eq!(original_len, root.len());
+
+                // after splitting the root has one element and two children, find which child the
+                // index is in
+                let first_child_len = root.children[0].len();
+                let (child, insertion_index) = if first_child_len < index {
+                    (&mut root.children[1], index - (first_child_len + 1))
+                } else {
+                    (&mut root.children[0], index)
+                };
+                root.length += 1;
+                child.insert_into_non_full_node(insertion_index, element)
+            } else {
+                root.insert_into_non_full_node(index, element)
+            }
+        } else {
+            self.root_node = Some(SequenceTreeNode {
+                elements: vec![element],
+                children: Vec::new(),
+                length: 1,
+            })
+        }
+        assert_eq!(self.len(), old_len + 1, "{:#?}", self);
+    }
+
+    /// Push the `element` onto the back of the sequence.
+    pub fn push(&mut self, element: T) {
+        let l = self.len();
+        self.insert(l, element)
+    }
+
+    /// Get the `element` at `index` in the sequence.
+    pub fn get(&self, index: usize) -> Option<&T> {
+        self.root_node.as_ref().and_then(|n| n.get(index))
+    }
+
+    /// Removes the element at `index` from the sequence.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `index` is out of bounds.
+    pub fn remove(&mut self, index: usize) -> T {
+        if let Some(root) = self.root_node.as_mut() {
+            #[cfg(debug_assertions)]
+            let len = root.check();
+            let old = root.remove(index);
+
+            if root.elements.is_empty() {
+                if root.is_leaf() {
+                    self.root_node = None;
+                } else {
+                    self.root_node = Some(root.children.remove(0));
+                }
+            }
+
+            #[cfg(debug_assertions)]
+            debug_assert_eq!(len, self.root_node.as_ref().map_or(0, |r| r.check()) + 1);
+            old
+        } else {
+            panic!("remove from empty tree")
+        }
+    }
+}
+
+impl<T> SequenceTreeNode<T>
+where
+    T: Clone + Debug,
+{
+    fn new() -> Self {
+        Self {
+            elements: Vec::new(),
+            children: Vec::new(),
+            length: 0,
+        }
+    }
+
+    fn len(&self) -> usize {
+        self.length
+    }
+
+    fn is_leaf(&self) -> bool {
+        self.children.is_empty()
+    }
+
+    fn is_full(&self) -> bool {
+        self.elements.len() >= 2 * B - 1
+    }
+
+    /// Returns the child index and the given index adjusted for the cumulative index before that
+    /// child.
+    fn find_child_index(&self, index: usize) -> (usize, usize) {
+        let mut cumulative_len = 0;
+        for (child_index, child) in self.children.iter().enumerate() {
+            if cumulative_len + child.len() >= index {
+                return (child_index, index - cumulative_len);
+            } else {
+                cumulative_len += child.len() + 1;
+            }
+        }
+        panic!("index not found in node")
+    }
+
+    fn insert_into_non_full_node(&mut self, index: usize, element: T) {
+        assert!(!self.is_full());
+        if self.is_leaf() {
+            self.length += 1;
+            self.elements.insert(index, element);
+        } else {
+            let (child_index, sub_index) = self.find_child_index(index);
+            let child = &mut self.children[child_index];
+
+            if child.is_full() {
+                self.split_child(child_index);
+
+                // child structure has changed so we need to find the index again
+                let (child_index, sub_index) = self.find_child_index(index);
+                let child = &mut self.children[child_index];
+                child.insert_into_non_full_node(sub_index, element);
+            } else {
+                child.insert_into_non_full_node(sub_index, element);
+            }
+            self.length += 1;
+        }
+    }
+
+    // A utility function to split the child `full_child_index` of this node
+    // Note that `full_child_index` must be full when this function is called.
+    fn split_child(&mut self, full_child_index: usize) {
+        let original_len_self = self.len();
+
+        // Create a new node which is going to store (B-1) keys
+        // of the full child.
+        let mut successor_sibling = SequenceTreeNode::new();
+
+        let full_child = &mut self.children[full_child_index];
+        let original_len = full_child.len();
+        assert!(full_child.is_full());
+
+        successor_sibling.elements = full_child.elements.split_off(B);
+
+        if !full_child.is_leaf() {
+            successor_sibling.children = full_child.children.split_off(B);
+        }
+
+        let middle = full_child.elements.pop().unwrap();
+
+        full_child.length =
+            full_child.elements.len() + full_child.children.iter().map(|c| c.len()).sum::<usize>();
+
+        successor_sibling.length = successor_sibling.elements.len()
+            + successor_sibling
+                .children
+                .iter()
+                .map(|c| c.len())
+                .sum::<usize>();
+
+        let z_len = successor_sibling.len();
+
+        let full_child_len = full_child.len();
+
+        self.children
+            .insert(full_child_index + 1, successor_sibling);
+
+        self.elements.insert(full_child_index, middle);
+
+        assert_eq!(full_child_len + z_len + 1, original_len, "{:#?}", self);
+
+        assert_eq!(original_len_self, self.len());
+    }
+
+    fn remove_from_leaf(&mut self, index: usize) -> T {
+        self.length -= 1;
+        self.elements.remove(index)
+    }
+
+    fn remove_element_from_non_leaf(&mut self, index: usize, element_index: usize) -> T {
+        self.length -= 1;
+        if self.children[element_index].elements.len() >= B {
+            let total_index = self.cumulative_index(element_index);
+            // recursively delete index - 1 in predecessor_node
+            let predecessor = self.children[element_index].remove(index - 1 - total_index);
+            // replace element with that one
+            mem::replace(&mut self.elements[element_index], predecessor)
+        } else if self.children[element_index + 1].elements.len() >= B {
+            // recursively delete index + 1 in successor_node
+            let total_index = self.cumulative_index(element_index + 1);
+            let successor = self.children[element_index + 1].remove(index + 1 - total_index);
+            // replace element with that one
+            mem::replace(&mut self.elements[element_index], successor)
+        } else {
+            let middle_element = self.elements.remove(element_index);
+            let successor_child = self.children.remove(element_index + 1);
+            self.children[element_index].merge(middle_element, successor_child);
+
+            let total_index = self.cumulative_index(element_index);
+            self.children[element_index].remove(index - total_index)
+        }
+    }
+
+    fn cumulative_index(&self, child_index: usize) -> usize {
+        self.children[0..child_index]
+            .iter()
+            .map(|c| c.len() + 1)
+            .sum()
+    }
+
+    fn remove_from_internal_child(&mut self, index: usize, mut child_index: usize) -> T {
+        if self.children[child_index].elements.len() < B
+            && if child_index > 0 {
+                self.children[child_index - 1].elements.len() < B
+            } else {
+                true
+            }
+            && if child_index + 1 < self.children.len() {
+                self.children[child_index + 1].elements.len() < B
+            } else {
+                true
+            }
+        {
+            // if the child and its immediate siblings have B-1 elements merge the child
+            // with one sibling, moving an element from this node into the new merged node
+            // to be the median
+
+            if child_index > 0 {
+                let middle = self.elements.remove(child_index - 1);
+
+                // use the predessor sibling
+                let successor = self.children.remove(child_index);
+                child_index -= 1;
+
+                self.children[child_index].merge(middle, successor);
+            } else {
+                let middle = self.elements.remove(child_index);
+
+                // use the sucessor sibling
+                let successor = self.children.remove(child_index + 1);
+
+                self.children[child_index].merge(middle, successor);
+            }
+        } else if self.children[child_index].elements.len() < B {
+            if child_index > 0
+                && self
+                    .children
+                    .get(child_index - 1)
+                    .map_or(false, |c| c.elements.len() >= B)
+            {
+                let last_element = self.children[child_index - 1].elements.pop().unwrap();
+                assert!(!self.children[child_index - 1].elements.is_empty());
+                self.children[child_index - 1].length -= 1;
+
+                let parent_element =
+                    mem::replace(&mut self.elements[child_index - 1], last_element);
+
+                self.children[child_index]
+                    .elements
+                    .insert(0, parent_element);
+                self.children[child_index].length += 1;
+
+                if let Some(last_child) = self.children[child_index - 1].children.pop() {
+                    self.children[child_index - 1].length -= last_child.len();
+                    self.children[child_index].length += last_child.len();
+                    self.children[child_index].children.insert(0, last_child);
+                }
+            } else if self
+                .children
+                .get(child_index + 1)
+                .map_or(false, |c| c.elements.len() >= B)
+            {
+                let first_element = self.children[child_index + 1].elements.remove(0);
+                self.children[child_index + 1].length -= 1;
+
+                assert!(!self.children[child_index + 1].elements.is_empty());
+
+                let parent_element = mem::replace(&mut self.elements[child_index], first_element);
+
+                self.children[child_index].length += 1;
+                self.children[child_index].elements.push(parent_element);
+
+                if !self.children[child_index + 1].is_leaf() {
+                    let first_child = self.children[child_index + 1].children.remove(0);
+                    self.children[child_index + 1].length -= first_child.len();
+                    self.children[child_index].length += first_child.len();
+
+                    self.children[child_index].children.push(first_child);
+                }
+            }
+        }
+        self.length -= 1;
+        let total_index = self.cumulative_index(child_index);
+        self.children[child_index].remove(index - total_index)
+    }
+
+    fn check(&self) -> usize {
+        let l = self.elements.len() + self.children.iter().map(|c| c.check()).sum::<usize>();
+        assert_eq!(self.len(), l, "{:#?}", self);
+
+        l
+    }
+
+    fn remove(&mut self, index: usize) -> T {
+        let original_len = self.len();
+        if self.is_leaf() {
+            let v = self.remove_from_leaf(index);
+            assert_eq!(original_len, self.len() + 1);
+            debug_assert_eq!(self.check(), self.len());
+            v
+        } else {
+            let mut total_index = 0;
+            for (child_index, child) in self.children.iter().enumerate() {
+                match (total_index + child.len()).cmp(&index) {
+                    Ordering::Less => {
+                        // should be later on in the loop
+                        total_index += child.len() + 1;
+                        continue;
+                    }
+                    Ordering::Equal => {
+                        let v = self.remove_element_from_non_leaf(
+                            index,
+                            min(child_index, self.elements.len() - 1),
+                        );
+                        assert_eq!(original_len, self.len() + 1);
+                        debug_assert_eq!(self.check(), self.len());
+                        return v;
+                    }
+                    Ordering::Greater => {
+                        let v = self.remove_from_internal_child(index, child_index);
+                        assert_eq!(original_len, self.len() + 1);
+                        debug_assert_eq!(self.check(), self.len());
+                        return v;
+                    }
+                }
+            }
+            panic!(
+                "index not found to remove {} {} {} {}",
+                index,
+                total_index,
+                self.len(),
+                self.check()
+            );
+        }
+    }
+
+    fn merge(&mut self, middle: T, successor_sibling: SequenceTreeNode<T>) {
+        self.elements.push(middle);
+        self.elements.extend(successor_sibling.elements);
+        self.children.extend(successor_sibling.children);
+        self.length += successor_sibling.length + 1;
+        assert!(self.is_full());
+    }
+
+    fn get(&self, index: usize) -> Option<&T> {
+        if self.is_leaf() {
+            return self.elements.get(index);
+        } else {
+            let mut cumulative_len = 0;
+            for (child_index, child) in self.children.iter().enumerate() {
+                match (cumulative_len + child.len()).cmp(&index) {
+                    Ordering::Less => {
+                        cumulative_len += child.len() + 1;
+                    }
+                    Ordering::Equal => return self.elements.get(child_index),
+                    Ordering::Greater => {
+                        return child.get(index - cumulative_len);
+                    }
+                }
+            }
+        }
+        None
+    }
+}
+
+impl<T> Default for SequenceTreeInternal<T>
+where
+    T: Clone + Debug,
+{
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl<T> PartialEq for SequenceTreeInternal<T>
+where
+    T: Clone + Debug + PartialEq,
+{
+    fn eq(&self, other: &Self) -> bool {
+        self.len() == other.len() && self.iter().zip(other.iter()).all(|(a, b)| a == b)
+    }
+}
+
+impl<'a, T> IntoIterator for &'a SequenceTreeInternal<T>
+where
+    T: Clone + Debug,
+{
+    type Item = &'a T;
+
+    type IntoIter = Iter<'a, T>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        Iter {
+            inner: self,
+            index: 0,
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct Iter<'a, T> {
+    inner: &'a SequenceTreeInternal<T>,
+    index: usize,
+}
+
+impl<'a, T> Iterator for Iter<'a, T>
+where
+    T: Clone + Debug,
+{
+    type Item = &'a T;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.index += 1;
+        self.inner.get(self.index - 1)
+    }
+
+    fn nth(&mut self, n: usize) -> Option<Self::Item> {
+        self.index += n + 1;
+        self.inner.get(self.index - 1)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use proptest::prelude::*;
+
+    use super::*;
+
+    #[test]
+    fn push_back() {
+        let mut t = SequenceTree::new();
+
+        t.push(1);
+        t.push(2);
+        t.push(3);
+        t.push(4);
+        t.push(5);
+        t.push(6);
+        t.push(8);
+        t.push(100);
+    }
+
+    #[test]
+    fn insert() {
+        let mut t = SequenceTree::new();
+
+        t.insert(0, 1);
+        t.insert(1, 1);
+        t.insert(0, 1);
+        t.insert(0, 1);
+        t.insert(0, 1);
+        t.insert(3, 1);
+        t.insert(4, 1);
+    }
+
+    #[test]
+    fn insert_book() {
+        let mut t = SequenceTree::new();
+
+        for i in 0..100 {
+            t.insert(i % 2, ());
+        }
+    }
+
+    #[test]
+    fn insert_book_vec() {
+        let mut t = SequenceTree::new();
+        let mut v = Vec::new();
+
+        for i in 0..100 {
+            t.insert(i % 3, ());
+            v.insert(i % 3, ());
+
+            assert_eq!(v, t.iter().copied().collect::<Vec<_>>())
+        }
+    }
+
+    fn arb_indices() -> impl Strategy<Value = Vec<usize>> {
+        proptest::collection::vec(any::<usize>(), 0..1000).prop_map(|v| {
+            let mut len = 0;
+            v.into_iter()
+                .map(|i| {
+                    len += 1;
+                    i % len
+                })
+                .collect::<Vec<_>>()
+        })
+    }
+
+    proptest! {
+
+        #[test]
+        fn proptest_insert(indices in arb_indices()) {
+            let mut t = SequenceTreeInternal::<usize>::new();
+            let mut v = Vec::new();
+
+            for i in indices{
+                if i <= v.len() {
+                    t.insert(i % 3, i);
+                    v.insert(i % 3, i);
+                } else {
+                    return Err(proptest::test_runner::TestCaseError::reject("index out of bounds"))
+                }
+
+                assert_eq!(v, t.iter().copied().collect::<Vec<_>>())
+            }
+        }
+
+    }
+
+    proptest! {
+
+        // This is a really slow test due to all the copying of the Vecs (i.e. not due to the
+        // sequencetree) so we only do a few runs
+        #![proptest_config(ProptestConfig::with_cases(20))]
+        #[test]
+        fn proptest_remove(inserts in arb_indices(), removes in arb_indices()) {
+            let mut t = SequenceTreeInternal::<usize>::new();
+            let mut v = Vec::new();
+
+            for i in inserts {
+                if i <= v.len() {
+                    t.insert(i , i);
+                    v.insert(i , i);
+                } else {
+                    return Err(proptest::test_runner::TestCaseError::reject("index out of bounds"))
+                }
+
+                assert_eq!(v, t.iter().copied().collect::<Vec<_>>())
+            }
+
+            for i in removes {
+                if i < v.len() {
+                    let tr = t.remove(i);
+                    let vr = v.remove(i);
+                    assert_eq!(tr, vr);
+                } else {
+                    return Err(proptest::test_runner::TestCaseError::reject("index out of bounds"))
+                }
+
+                assert_eq!(v, t.iter().copied().collect::<Vec<_>>())
+            }
+        }
+
+    }
+}
diff --git a/rust/automerge/src/storage/change.rs b/rust/automerge/src/storage/change.rs
index 61db0b00..a420da39 100644
--- a/rust/automerge/src/storage/change.rs
+++ b/rust/automerge/src/storage/change.rs
@@ -437,6 +437,8 @@ pub(crate) trait AsChangeOp<'a> {
     fn action(&self) -> u64;
     fn val(&self) -> Cow<'a, ScalarValue>;
     fn pred(&self) -> Self::PredIter;
+    fn expand(&self) -> bool;
+    fn mark_name(&self) -> Option<Cow<'a, smol_str::SmolStr>>;
 }
 
 impl ChangeBuilder<Set<NonZeroU64>, Set<ActorId>, Set<u64>, Set<i64>> {
diff --git a/rust/automerge/src/storage/change/change_actors.rs b/rust/automerge/src/storage/change/change_actors.rs
index 61f1221d..951201dd 100644
--- a/rust/automerge/src/storage/change/change_actors.rs
+++ b/rust/automerge/src/storage/change/change_actors.rs
@@ -1,4 +1,7 @@
-use std::collections::{BTreeMap, BTreeSet};
+use std::{
+    borrow::Cow,
+    collections::{BTreeMap, BTreeSet},
+};
 
 use crate::convert;
 
@@ -244,6 +247,14 @@ where
     fn val(&self) -> std::borrow::Cow<'aschangeop, crate::ScalarValue> {
         self.op.val()
     }
+
+    fn expand(&self) -> bool {
+        self.op.expand()
+    }
+
+    fn mark_name(&self) -> Option<Cow<'aschangeop, smol_str::SmolStr>> {
+        self.op.mark_name()
+    }
 }
 
 pub(crate) struct WithChangeActorsPredIter<'actors, 'aschangeop, A, I, O, C, P> {
diff --git a/rust/automerge/src/storage/change/change_op_columns.rs b/rust/automerge/src/storage/change/change_op_columns.rs
index 86ec59c2..145073e6 100644
--- a/rust/automerge/src/storage/change/change_op_columns.rs
+++ b/rust/automerge/src/storage/change/change_op_columns.rs
@@ -1,16 +1,16 @@
-use std::{convert::TryFrom, ops::Range};
+use std::{borrow::Cow, convert::TryFrom, ops::Range};
 
 use crate::{
     columnar::{
         column_range::{
             generic::{GenericColumnRange, GroupRange, GroupedColumnRange, SimpleColRange},
-            BooleanRange, DeltaRange, Key, KeyEncoder, KeyIter, KeyRange, ObjIdEncoder, ObjIdIter,
-            ObjIdRange, OpIdListEncoder, OpIdListIter, OpIdListRange, RleRange, ValueEncoder,
-            ValueIter, ValueRange,
+            BooleanRange, DeltaRange, Key, KeyEncoder, KeyIter, KeyRange, MaybeBooleanRange,
+            ObjIdEncoder, ObjIdIter, ObjIdRange, OpIdListEncoder, OpIdListIter, OpIdListRange,
+            RleRange, ValueEncoder, ValueIter, ValueRange,
         },
         encoding::{
-            BooleanDecoder, BooleanEncoder, ColumnDecoder, DecodeColumnError, RleDecoder,
-            RleEncoder,
+            BooleanDecoder, BooleanEncoder, ColumnDecoder, DecodeColumnError, MaybeBooleanDecoder,
+            MaybeBooleanEncoder, RleDecoder, RleEncoder,
         },
     },
     convert,
@@ -32,6 +32,8 @@ const INSERT_COL_ID: ColumnId = ColumnId::new(3);
 const ACTION_COL_ID: ColumnId = ColumnId::new(4);
 const VAL_COL_ID: ColumnId = ColumnId::new(5);
 const PRED_COL_ID: ColumnId = ColumnId::new(7);
+const EXPAND_COL_ID: ColumnId = ColumnId::new(8);
+const MARK_NAME_COL_ID: ColumnId = ColumnId::new(9);
 
 #[derive(Clone, Debug, PartialEq)]
 pub(crate) struct ChangeOp {
@@ -41,6 +43,8 @@ pub(crate) struct ChangeOp {
     pub(crate) pred: Vec<OpId>,
     pub(crate) action: u64,
     pub(crate) obj: ObjId,
+    pub(crate) expand: bool,
+    pub(crate) mark_name: Option<smol_str::SmolStr>,
 }
 
 impl<'a, A: AsChangeOp<'a, ActorId = usize, OpId = OpId>> From<A> for ChangeOp {
@@ -59,6 +63,8 @@ impl<'a, A: AsChangeOp<'a, ActorId = usize, OpId = OpId>> From<A> for ChangeOp {
             pred: a.pred().collect(),
             insert: a.insert(),
             action: a.action(),
+            expand: a.expand(),
+            mark_name: a.mark_name().map(|n| n.into_owned()),
         }
     }
 }
@@ -99,6 +105,14 @@ impl<'a> AsChangeOp<'a> for &'a ChangeOp {
     fn action(&self) -> u64 {
         self.action
     }
+
+    fn expand(&self) -> bool {
+        self.expand
+    }
+
+    fn mark_name(&self) -> Option<Cow<'a, smol_str::SmolStr>> {
+        self.mark_name.as_ref().map(Cow::Borrowed)
+    }
 }
 
 #[derive(Clone, Debug, PartialEq)]
@@ -109,6 +123,8 @@ pub(crate) struct ChangeOpsColumns {
     action: RleRange<u64>,
     val: ValueRange,
     pred: OpIdListRange,
+    expand: MaybeBooleanRange,
+    mark_name: RleRange<smol_str::SmolStr>,
 }
 
 impl ChangeOpsColumns {
@@ -121,6 +137,8 @@ impl ChangeOpsColumns {
             action: self.action.decoder(data),
             val: self.val.iter(data),
             pred: self.pred.iter(data),
+            expand: self.expand.decoder(data),
+            mark_name: self.mark_name.decoder(data),
         }
     }
 
@@ -147,12 +165,16 @@ impl ChangeOpsColumns {
         Op: convert::OpId<usize> + 'a,
         C: AsChangeOp<'c, OpId = Op> + 'a,
     {
+        tracing::trace!(expands = ?ops.clone().map(|op| op.expand()).collect::<Vec<_>>(), "encoding change ops");
         let obj = ObjIdRange::encode(ops.clone().map(|o| o.obj()), out);
         let key = KeyRange::encode(ops.clone().map(|o| o.key()), out);
         let insert = BooleanRange::encode(ops.clone().map(|o| o.insert()), out);
         let action = RleRange::encode(ops.clone().map(|o| Some(o.action())), out);
         let val = ValueRange::encode(ops.clone().map(|o| o.val()), out);
-        let pred = OpIdListRange::encode(ops.map(|o| o.pred()), out);
+        let pred = OpIdListRange::encode(ops.clone().map(|o| o.pred()), out);
+        let expand = MaybeBooleanRange::encode(ops.clone().map(|o| o.expand()), out);
+        let mark_name =
+            RleRange::encode::<Cow<'_, smol_str::SmolStr>, _>(ops.map(|o| o.mark_name()), out);
         Self {
             obj,
             key,
@@ -160,6 +182,8 @@ impl ChangeOpsColumns {
             action,
             val,
             pred,
+            expand,
+            mark_name,
         }
     }
 
@@ -175,13 +199,18 @@ impl ChangeOpsColumns {
         let mut action = RleEncoder::<_, u64>::from(Vec::new());
         let mut val = ValueEncoder::new();
         let mut pred = OpIdListEncoder::new();
+        let mut expand = MaybeBooleanEncoder::new();
+        let mut mark_name = RleEncoder::<_, smol_str::SmolStr>::new(Vec::new());
         for op in ops {
+            tracing::trace!(expand=?op.expand(), "expand");
             obj.append(op.obj());
             key.append(op.key());
             insert.append(op.insert());
             action.append_value(op.action());
             val.append(&op.val());
             pred.append(op.pred());
+            expand.append(op.expand());
+            mark_name.append(op.mark_name());
         }
         let obj = obj.finish(out);
         let key = key.finish(out);
@@ -199,6 +228,16 @@ impl ChangeOpsColumns {
         let val = val.finish(out);
         let pred = pred.finish(out);
 
+        let expand_start = out.len();
+        let (expand, _) = expand.finish();
+        out.extend(expand);
+        let expand = MaybeBooleanRange::from(expand_start..out.len());
+
+        let mark_name_start = out.len();
+        let (mark_name, _) = mark_name.finish();
+        out.extend(mark_name);
+        let mark_name = RleRange::from(mark_name_start..out.len());
+
         Self {
             obj,
             key,
@@ -206,6 +245,8 @@ impl ChangeOpsColumns {
             action,
             val,
             pred,
+            expand,
+            mark_name,
         }
     }
 
@@ -272,6 +313,18 @@ impl ChangeOpsColumns {
                 ),
             ]);
         }
+        if !self.expand.is_empty() {
+            cols.push(RawColumn::new(
+                ColumnSpec::new(EXPAND_COL_ID, ColumnType::Boolean, false),
+                self.expand.clone().into(),
+            ));
+        }
+        if !self.mark_name.is_empty() {
+            cols.push(RawColumn::new(
+                ColumnSpec::new(MARK_NAME_COL_ID, ColumnType::String, false),
+                self.mark_name.clone().into(),
+            ));
+        }
         cols.into_iter().collect()
     }
 }
@@ -296,6 +349,8 @@ pub(crate) struct ChangeOpsIter<'a> {
     action: RleDecoder<'a, u64>,
     val: ValueIter<'a>,
     pred: OpIdListIter<'a>,
+    expand: MaybeBooleanDecoder<'a>,
+    mark_name: RleDecoder<'a, smol_str::SmolStr>,
 }
 
 impl<'a> ChangeOpsIter<'a> {
@@ -317,6 +372,8 @@ impl<'a> ChangeOpsIter<'a> {
             let action = self.action.next_in_col("action")?;
             let val = self.val.next_in_col("value")?;
             let pred = self.pred.next_in_col("pred")?;
+            let expand = self.expand.maybe_next_in_col("expand")?.unwrap_or(false);
+            let mark_name = self.mark_name.maybe_next_in_col("mark_name")?;
 
             // This check is necessary to ensure that OpType::from_action_and_value
             // cannot panic later in the process.
@@ -329,6 +386,8 @@ impl<'a> ChangeOpsIter<'a> {
                 action,
                 val,
                 pred,
+                expand,
+                mark_name,
             }))
         }
     }
@@ -375,6 +434,8 @@ impl TryFrom<Columns> for ChangeOpsColumns {
         let mut pred_group: Option<RleRange<u64>> = None;
         let mut pred_actor: Option<RleRange<u64>> = None;
         let mut pred_ctr: Option<DeltaRange> = None;
+        let mut expand: Option<MaybeBooleanRange> = None;
+        let mut mark_name: Option<RleRange<smol_str::SmolStr>> = None;
         let mut other = Columns::empty();
 
         for (index, col) in columns.into_iter().enumerate() {
@@ -429,6 +490,8 @@ impl TryFrom<Columns> for ChangeOpsColumns {
                     }
                     _ => return Err(ParseChangeColumnsError::MismatchingColumn { index }),
                 },
+                (EXPAND_COL_ID, ColumnType::Boolean) => expand = Some(col.range().into()),
+                (MARK_NAME_COL_ID, ColumnType::String) => mark_name = Some(col.range().into()),
                 (other_type, other_col) => {
                     tracing::warn!(typ=?other_type, id=?other_col, "unknown column");
                     other.append(col);
@@ -454,6 +517,8 @@ impl TryFrom<Columns> for ChangeOpsColumns {
             action: action.unwrap_or(0..0).into(),
             val: val.unwrap_or_else(|| ValueRange::new((0..0).into(), (0..0).into())),
             pred,
+            expand: expand.unwrap_or_else(|| (0..0).into()),
+            mark_name: mark_name.unwrap_or_else(|| (0..0).into()),
         })
     }
 }
@@ -471,6 +536,8 @@ mod tests {
                      pred in proptest::collection::vec(opid(), 0..20),
                      action in 0_u64..6,
                      obj in opid(),
+                     mark_name in proptest::option::of(any::<String>().prop_map(|s| s.into())),
+                     expand in any::<bool>(),
                      insert in any::<bool>()) -> ChangeOp {
 
                     let val = if action == 5 && !(value.is_int() || value.is_uint()) {
@@ -483,6 +550,8 @@ mod tests {
                 pred,
                 action,
                 insert,
+                expand,
+                mark_name,
             }
         }
     }
diff --git a/rust/automerge/src/storage/convert/op_as_changeop.rs b/rust/automerge/src/storage/convert/op_as_changeop.rs
index 00b5e940..c08fa063 100644
--- a/rust/automerge/src/storage/convert/op_as_changeop.rs
+++ b/rust/automerge/src/storage/convert/op_as_changeop.rs
@@ -5,7 +5,7 @@ use crate::{
     convert,
     op_set::OpSetMetadata,
     storage::AsChangeOp,
-    types::{ActorId, Key, ObjId, Op, OpId, OpType, ScalarValue},
+    types::{ActorId, Key, MarkData, ObjId, Op, OpId, OpType, ScalarValue},
 };
 
 /// Wrap an op in an implementation of `AsChangeOp` which represents actor IDs using a reference to
@@ -93,9 +93,12 @@ impl<'a> AsChangeOp<'a> for OpWithMetadata<'a> {
 
     fn val(&self) -> Cow<'a, ScalarValue> {
         match &self.op.action {
-            OpType::Make(..) | OpType::Delete => Cow::Owned(ScalarValue::Null),
+            OpType::Make(..) | OpType::Delete | OpType::MarkEnd(..) => {
+                Cow::Owned(ScalarValue::Null)
+            }
             OpType::Increment(i) => Cow::Owned(ScalarValue::Int(*i)),
             OpType::Put(s) => Cow::Borrowed(s),
+            OpType::MarkBegin(_, MarkData { value, .. }) => Cow::Borrowed(value),
         }
     }
 
@@ -125,4 +128,19 @@ impl<'a> AsChangeOp<'a> for OpWithMetadata<'a> {
             Key::Seq(e) => convert::Key::Elem(convert::ElemId::Op(self.wrap(&e.0))),
         }
     }
+
+    fn expand(&self) -> bool {
+        matches!(
+            self.op.action,
+            OpType::MarkBegin(true, _) | OpType::MarkEnd(true)
+        )
+    }
+
+    fn mark_name(&self) -> Option<Cow<'a, smol_str::SmolStr>> {
+        if let OpType::MarkBegin(_, MarkData { name, .. }) = &self.op.action {
+            Some(Cow::Owned(name.clone()))
+        } else {
+            None
+        }
+    }
 }
diff --git a/rust/automerge/src/storage/convert/op_as_docop.rs b/rust/automerge/src/storage/convert/op_as_docop.rs
index 8d237354..09598b67 100644
--- a/rust/automerge/src/storage/convert/op_as_docop.rs
+++ b/rust/automerge/src/storage/convert/op_as_docop.rs
@@ -4,7 +4,7 @@ use crate::{
     convert,
     indexed_cache::IndexedCache,
     storage::AsDocOp,
-    types::{ElemId, Key, ObjId, Op, OpId, OpType, ScalarValue},
+    types::{ElemId, Key, MarkData, ObjId, Op, OpId, OpType, ScalarValue},
 };
 
 /// Create an [`AsDocOp`] implementation for a [`crate::types::Op`]
@@ -90,6 +90,7 @@ impl<'a> AsDocOp<'a> for OpAsDocOp<'a> {
         match &self.op.action {
             OpType::Put(v) => Cow::Borrowed(v),
             OpType::Increment(i) => Cow::Owned(ScalarValue::Int(*i)),
+            OpType::MarkBegin(_, MarkData { value, .. }) => Cow::Borrowed(value),
             _ => Cow::Owned(ScalarValue::Null),
         }
     }
@@ -109,6 +110,22 @@ impl<'a> AsDocOp<'a> for OpAsDocOp<'a> {
     fn action(&self) -> u64 {
         self.op.action.action_index()
     }
+
+    fn expand(&self) -> bool {
+        if let OpType::MarkBegin(expand, _) | OpType::MarkEnd(expand) = &self.op.action {
+            *expand
+        } else {
+            false
+        }
+    }
+
+    fn mark_name(&self) -> Option<Cow<'a, smol_str::SmolStr>> {
+        if let OpType::MarkBegin(_, MarkData { name, .. }) = &self.op.action {
+            Some(Cow::Owned(name.clone()))
+        } else {
+            None
+        }
+    }
 }
 
 pub(crate) struct OpAsDocOpSuccIter<'a> {
diff --git a/rust/automerge/src/storage/document/doc_op_columns.rs b/rust/automerge/src/storage/document/doc_op_columns.rs
index 82de17eb..7248b1b3 100644
--- a/rust/automerge/src/storage/document/doc_op_columns.rs
+++ b/rust/automerge/src/storage/document/doc_op_columns.rs
@@ -4,13 +4,13 @@ use crate::{
     columnar::{
         column_range::{
             generic::{GenericColumnRange, GroupRange, GroupedColumnRange, SimpleColRange},
-            BooleanRange, DeltaRange, Key, KeyEncoder, KeyIter, KeyRange, ObjIdEncoder, ObjIdIter,
-            ObjIdRange, OpIdEncoder, OpIdIter, OpIdListEncoder, OpIdListIter, OpIdListRange,
-            OpIdRange, RleRange, ValueEncoder, ValueIter, ValueRange,
+            BooleanRange, DeltaRange, Key, KeyEncoder, KeyIter, KeyRange, MaybeBooleanRange,
+            ObjIdEncoder, ObjIdIter, ObjIdRange, OpIdEncoder, OpIdIter, OpIdListEncoder,
+            OpIdListIter, OpIdListRange, OpIdRange, RleRange, ValueEncoder, ValueIter, ValueRange,
         },
         encoding::{
-            BooleanDecoder, BooleanEncoder, ColumnDecoder, DecodeColumnError, RleDecoder,
-            RleEncoder,
+            BooleanDecoder, BooleanEncoder, ColumnDecoder, DecodeColumnError, MaybeBooleanDecoder,
+            MaybeBooleanEncoder, RleDecoder, RleEncoder,
         },
     },
     convert,
@@ -28,6 +28,8 @@ const INSERT_COL_ID: ColumnId = ColumnId::new(3);
 const ACTION_COL_ID: ColumnId = ColumnId::new(4);
 const VAL_COL_ID: ColumnId = ColumnId::new(5);
 const SUCC_COL_ID: ColumnId = ColumnId::new(8);
+const EXPAND_COL_ID: ColumnId = ColumnId::new(9);
+const MARK_NAME_COL_ID: ColumnId = ColumnId::new(10);
 
 /// The form operations take in the compressed document format.
 #[derive(Debug)]
@@ -36,9 +38,11 @@ pub(crate) struct DocOp {
     pub(crate) object: ObjId,
     pub(crate) key: Key,
     pub(crate) insert: bool,
-    pub(crate) action: usize,
+    pub(crate) action: u64,
     pub(crate) value: ScalarValue,
     pub(crate) succ: Vec<OpId>,
+    pub(crate) expand: bool,
+    pub(crate) mark_name: Option<smol_str::SmolStr>,
 }
 
 #[derive(Debug, Clone)]
@@ -52,6 +56,8 @@ pub(crate) struct DocOpColumns {
     succ: OpIdListRange,
     #[allow(dead_code)]
     other: Columns,
+    expand: MaybeBooleanRange,
+    mark_name: RleRange<smol_str::SmolStr>,
 }
 
 struct DocId {
@@ -90,6 +96,8 @@ pub(crate) trait AsDocOp<'a> {
     fn action(&self) -> u64;
     fn val(&self) -> Cow<'a, ScalarValue>;
     fn succ(&self) -> Self::SuccIter;
+    fn expand(&self) -> bool;
+    fn mark_name(&self) -> Option<Cow<'a, smol_str::SmolStr>>;
 }
 
 impl DocOpColumns {
@@ -118,7 +126,9 @@ impl DocOpColumns {
         let insert = BooleanRange::encode(ops.clone().map(|o| o.insert()), out);
         let action = RleRange::encode(ops.clone().map(|o| Some(o.action())), out);
         let val = ValueRange::encode(ops.clone().map(|o| o.val()), out);
-        let succ = OpIdListRange::encode(ops.map(|o| o.succ()), out);
+        let succ = OpIdListRange::encode(ops.clone().map(|o| o.succ()), out);
+        let expand = MaybeBooleanRange::encode(ops.clone().map(|o| o.expand()), out);
+        let mark_name = RleRange::encode(ops.map(|o| o.mark_name()), out);
         Self {
             obj,
             key,
@@ -127,6 +137,8 @@ impl DocOpColumns {
             action,
             val,
             succ,
+            expand,
+            mark_name,
             other: Columns::empty(),
         }
     }
@@ -144,6 +156,8 @@ impl DocOpColumns {
         let mut action = RleEncoder::<_, u64>::from(Vec::new());
         let mut val = ValueEncoder::new();
         let mut succ = OpIdListEncoder::new();
+        let mut expand = MaybeBooleanEncoder::new();
+        let mut mark_name = RleEncoder::<_, smol_str::SmolStr>::new(Vec::new());
         for op in ops {
             obj.append(op.obj());
             key.append(op.key());
@@ -152,6 +166,8 @@ impl DocOpColumns {
             action.append(Some(op.action()));
             val.append(&op.val());
             succ.append(op.succ());
+            expand.append(op.expand());
+            mark_name.append(op.mark_name());
         }
         let obj = obj.finish(out);
         let key = key.finish(out);
@@ -169,6 +185,17 @@ impl DocOpColumns {
 
         let val = val.finish(out);
         let succ = succ.finish(out);
+
+        let expand_start = out.len();
+        let (expand_out, _) = expand.finish();
+        out.extend(expand_out);
+        let expand = MaybeBooleanRange::from(expand_start..out.len());
+
+        let mark_name_start = out.len();
+        let (mark_name_out, _) = mark_name.finish();
+        out.extend(mark_name_out);
+        let mark_name = RleRange::from(mark_name_start..out.len());
+
         DocOpColumns {
             obj,
             key,
@@ -177,6 +204,8 @@ impl DocOpColumns {
             action,
             val,
             succ,
+            expand,
+            mark_name,
             other: Columns::empty(),
         }
     }
@@ -190,6 +219,8 @@ impl DocOpColumns {
             insert: self.insert.decoder(data),
             value: self.val.iter(data),
             succ: self.succ.iter(data),
+            expand: self.expand.decoder(data),
+            mark_name: self.mark_name.decoder(data),
         }
     }
 
@@ -264,6 +295,18 @@ impl DocOpColumns {
                 ),
             ]);
         }
+        if !self.expand.is_empty() {
+            cols.push(RawColumn::new(
+                ColumnSpec::new(EXPAND_COL_ID, ColumnType::Boolean, false),
+                self.expand.clone().into(),
+            ));
+        }
+        if !self.mark_name.is_empty() {
+            cols.push(RawColumn::new(
+                ColumnSpec::new(MARK_NAME_COL_ID, ColumnType::String, false),
+                self.mark_name.clone().into(),
+            ));
+        }
         cols.into_iter().collect()
     }
 }
@@ -277,6 +320,8 @@ pub(crate) struct DocOpColumnIter<'a> {
     insert: BooleanDecoder<'a>,
     value: ValueIter<'a>,
     succ: OpIdListIter<'a>,
+    expand: MaybeBooleanDecoder<'a>,
+    mark_name: RleDecoder<'a, smol_str::SmolStr>,
 }
 
 impl<'a> DocOpColumnIter<'a> {
@@ -321,14 +366,18 @@ impl<'a> DocOpColumnIter<'a> {
             let value = self.value.next_in_col("value")?;
             let succ = self.succ.next_in_col("succ")?;
             let insert = self.insert.next_in_col("insert")?;
+            let expand = self.expand.maybe_next_in_col("expand")?.unwrap_or(false);
+            let mark_name = self.mark_name.maybe_next_in_col("mark_name")?;
             Ok(Some(DocOp {
                 id,
                 value,
-                action: action as usize,
+                action,
                 object: obj,
                 key,
                 succ,
                 insert,
+                expand,
+                mark_name,
             }))
         }
     }
@@ -363,6 +412,8 @@ impl TryFrom<Columns> for DocOpColumns {
         let mut succ_group: Option<RleRange<u64>> = None;
         let mut succ_actor: Option<RleRange<u64>> = None;
         let mut succ_ctr: Option<DeltaRange> = None;
+        let mut expand: Option<MaybeBooleanRange> = None;
+        let mut mark_name: Option<RleRange<smol_str::SmolStr>> = None;
         let mut other = Columns::empty();
 
         for (index, col) in columns.into_iter().enumerate() {
@@ -416,6 +467,8 @@ impl TryFrom<Columns> for DocOpColumns {
                     }
                     _ => return Err(Error::MismatchingColumn { index }),
                 },
+                (EXPAND_COL_ID, ColumnType::Boolean) => expand = Some(col.range().into()),
+                (MARK_NAME_COL_ID, ColumnType::String) => mark_name = Some(col.range().into()),
                 (other_col, other_type) => {
                     tracing::warn!(id=?other_col, typ=?other_type, "unknown column type");
                     other.append(col)
@@ -444,6 +497,8 @@ impl TryFrom<Columns> for DocOpColumns {
                 succ_actor.unwrap_or_else(|| (0..0).into()),
                 succ_ctr.unwrap_or_else(|| (0..0).into()),
             ),
+            expand: expand.unwrap_or_else(|| (0..0).into()),
+            mark_name: mark_name.unwrap_or_else(|| (0..0).into()),
             other,
         })
     }
diff --git a/rust/automerge/src/storage/load/reconstruct_document.rs b/rust/automerge/src/storage/load/reconstruct_document.rs
index 44ace72a..1f80a85d 100644
--- a/rust/automerge/src/storage/load/reconstruct_document.rs
+++ b/rust/automerge/src/storage/load/reconstruct_document.rs
@@ -17,8 +17,6 @@ pub(crate) enum Error {
     OpsOutOfOrder,
     #[error("error reading operation: {0:?}")]
     ReadOp(Box<dyn std::error::Error + Send + Sync + 'static>),
-    #[error("an operation contained an invalid action")]
-    InvalidAction,
     #[error("an operation referenced a missing actor id")]
     MissingActor,
     #[error("invalid changes: {0}")]
@@ -29,6 +27,8 @@ pub(crate) enum Error {
     MissingOps,
     #[error("succ out of order")]
     SuccOutOfOrder,
+    #[error(transparent)]
+    InvalidOp(#[from] crate::error::InvalidOpType),
 }
 
 pub(crate) struct MismatchedHeads {
@@ -345,9 +345,10 @@ fn import_op(m: &mut OpSetMetadata, op: DocOp) -> Result<Op, Error> {
             return Err(Error::MissingActor);
         }
     }
+    let action = OpType::from_action_and_value(op.action, op.value, op.mark_name, op.expand);
     Ok(Op {
         id: check_opid(m, op.id)?,
-        action: parse_optype(op.action, op.value)?,
+        action,
         key,
         succ: m.try_sorted_opids(op.succ).ok_or(Error::SuccOutOfOrder)?,
         pred: OpIds::empty(),
@@ -367,25 +368,3 @@ fn check_opid(m: &OpSetMetadata, opid: OpId) -> Result<OpId, Error> {
         }
     }
 }
-
-fn parse_optype(action_index: usize, value: ScalarValue) -> Result<OpType, Error> {
-    match action_index {
-        0 => Ok(OpType::Make(ObjType::Map)),
-        1 => Ok(OpType::Put(value)),
-        2 => Ok(OpType::Make(ObjType::List)),
-        3 => Ok(OpType::Delete),
-        4 => Ok(OpType::Make(ObjType::Text)),
-        5 => match value {
-            ScalarValue::Int(i) => Ok(OpType::Increment(i)),
-            _ => {
-                tracing::error!(?value, "invalid value for counter op");
-                Err(Error::InvalidAction)
-            }
-        },
-        6 => Ok(OpType::Make(ObjType::Table)),
-        other => {
-            tracing::error!(action = other, "unknown action type");
-            Err(Error::InvalidAction)
-        }
-    }
-}
diff --git a/rust/automerge/src/transaction/inner.rs b/rust/automerge/src/transaction/inner.rs
index 0fe735d5..5afd2e1d 100644
--- a/rust/automerge/src/transaction/inner.rs
+++ b/rust/automerge/src/transaction/inner.rs
@@ -1,6 +1,7 @@
 use std::num::NonZeroU64;
 
 use crate::exid::ExId;
+use crate::marks::{ExpandMark, Mark};
 use crate::query::{self, OpIdSearch};
 use crate::storage::Change as StoredChange;
 use crate::types::{Key, ListEncoding, ObjId, OpId, OpIds, TextEncoding};
@@ -638,7 +639,7 @@ impl TransactionInner {
                         for (offset, v) in values.iter().enumerate() {
                             let op = &self.operations[start + offset].1;
                             let value = (v.clone().into(), doc.ops().id_to_exid(op.id));
-                            obs.insert(doc, ex_obj.clone(), index + offset, value)
+                            obs.insert(doc, ex_obj.clone(), index + offset, value, false)
                         }
                     }
                 }
@@ -648,6 +649,51 @@ impl TransactionInner {
         Ok(())
     }
 
+    pub(crate) fn mark<Obs: OpObserver>(
+        &mut self,
+        doc: &mut Automerge,
+        op_observer: Option<&mut Obs>,
+        ex_obj: &ExId,
+        mark: Mark<'_>,
+        expand: ExpandMark,
+    ) -> Result<(), AutomergeError> {
+        let (obj, _obj_type) = doc.exid_to_obj(ex_obj)?;
+        if let Some(obs) = op_observer {
+            let action = OpType::MarkBegin(expand.left(), mark.data.clone().into_owned());
+            self.do_insert(doc, Some(obs), obj, mark.start, action)?;
+            self.do_insert(
+                doc,
+                Some(obs),
+                obj,
+                mark.end,
+                OpType::MarkEnd(expand.right()),
+            )?;
+            if mark.value().is_null() {
+                obs.unmark(doc, ex_obj.clone(), mark.name(), mark.start, mark.end);
+            } else {
+                obs.mark(doc, ex_obj.clone(), Some(mark).into_iter())
+            }
+        } else {
+            let action = OpType::MarkBegin(expand.left(), mark.data.into_owned());
+            self.do_insert::<Obs>(doc, None, obj, mark.start, action)?;
+            self.do_insert::<Obs>(doc, None, obj, mark.end, OpType::MarkEnd(expand.right()))?;
+        }
+        Ok(())
+    }
+
+    pub(crate) fn unmark<Obs: OpObserver>(
+        &mut self,
+        doc: &mut Automerge,
+        op_observer: Option<&mut Obs>,
+        ex_obj: &ExId,
+        name: &str,
+        start: usize,
+        end: usize,
+    ) -> Result<(), AutomergeError> {
+        let mark = Mark::new(name.to_string(), ScalarValue::Null, start, end);
+        self.mark(doc, op_observer, ex_obj, mark, ExpandMark::None)
+    }
+
     fn finalize_op<Obs: OpObserver>(
         &mut self,
         doc: &mut Automerge,
@@ -660,23 +706,24 @@ impl TransactionInner {
         if let Some(op_observer) = op_observer {
             let ex_obj = doc.ops().id_to_exid(obj.0);
             if op.insert {
-                let obj_type = doc.ops().object_type(&obj);
-                assert!(obj_type.unwrap().is_sequence());
-                match (obj_type, prop) {
-                    (Some(ObjType::List), Prop::Seq(index)) => {
-                        let value = (op.value(), doc.ops().id_to_exid(op.id));
-                        op_observer.insert(doc, ex_obj, index, value)
-                    }
-                    (Some(ObjType::Text), Prop::Seq(index)) => {
-                        // FIXME
-                        if op_observer.text_as_seq() {
+                if !op.is_mark() {
+                    let obj_type = doc.ops().object_type(&obj);
+                    assert!(obj_type.unwrap().is_sequence());
+                    match (obj_type, prop) {
+                        (Some(ObjType::List), Prop::Seq(index)) => {
                             let value = (op.value(), doc.ops().id_to_exid(op.id));
-                            op_observer.insert(doc, ex_obj, index, value)
-                        } else {
-                            op_observer.splice_text(doc, ex_obj, index, op.to_str())
+                            op_observer.insert(doc, ex_obj, index, value, false)
                         }
+                        (Some(ObjType::Text), Prop::Seq(index)) => {
+                            if op_observer.text_as_seq() {
+                                let value = (op.value(), doc.ops().id_to_exid(op.id));
+                                op_observer.insert(doc, ex_obj, index, value, false)
+                            } else {
+                                op_observer.splice_text(doc, ex_obj, index, op.to_str())
+                            }
+                        }
+                        _ => {}
                     }
-                    _ => {}
                 }
             } else if op.is_delete() {
                 op_observer.delete(doc, ex_obj, prop);
diff --git a/rust/automerge/src/transaction/manual_transaction.rs b/rust/automerge/src/transaction/manual_transaction.rs
index fa5f6340..52fdd0d7 100644
--- a/rust/automerge/src/transaction/manual_transaction.rs
+++ b/rust/automerge/src/transaction/manual_transaction.rs
@@ -1,6 +1,7 @@
 use std::ops::RangeBounds;
 
 use crate::exid::ExId;
+use crate::marks::{ExpandMark, Mark};
 use crate::op_observer::BranchableObserver;
 use crate::{
     Automerge, ChangeHash, KeysAt, ObjType, OpObserver, Prop, ReadDoc, ScalarValue, Value, Values,
@@ -190,6 +191,18 @@ impl<'a, Obs: observation::Observation> ReadDoc for Transaction<'a, Obs> {
         self.doc.text_at(obj, heads)
     }
 
+    fn marks<O: AsRef<ExId>>(&self, obj: O) -> Result<Vec<Mark<'_>>, AutomergeError> {
+        self.doc.marks(obj)
+    }
+
+    fn marks_at<O: AsRef<ExId>>(
+        &self,
+        obj: O,
+        heads: &[ChangeHash],
+    ) -> Result<Vec<Mark<'_>>, AutomergeError> {
+        self.doc.marks_at(obj, heads)
+    }
+
     fn get<O: AsRef<ExId>, P: Into<Prop>>(
         &self,
         obj: O,
@@ -330,6 +343,25 @@ impl<'a, Obs: observation::Observation> Transactable for Transaction<'a, Obs> {
         self.do_tx(|tx, doc, obs| tx.splice_text(doc, obs, obj.as_ref(), pos, del, text))
     }
 
+    fn mark<O: AsRef<ExId>>(
+        &mut self,
+        obj: O,
+        mark: Mark<'_>,
+        expand: ExpandMark,
+    ) -> Result<(), AutomergeError> {
+        self.do_tx(|tx, doc, obs| tx.mark(doc, obs, obj.as_ref(), mark, expand))
+    }
+
+    fn unmark<O: AsRef<ExId>>(
+        &mut self,
+        obj: O,
+        name: &str,
+        start: usize,
+        end: usize,
+    ) -> Result<(), AutomergeError> {
+        self.do_tx(|tx, doc, obs| tx.unmark(doc, obs, obj.as_ref(), name, start, end))
+    }
+
     fn base_heads(&self) -> Vec<ChangeHash> {
         self.doc.get_heads()
     }
diff --git a/rust/automerge/src/transaction/transactable.rs b/rust/automerge/src/transaction/transactable.rs
index 05c48c79..81b83dff 100644
--- a/rust/automerge/src/transaction/transactable.rs
+++ b/rust/automerge/src/transaction/transactable.rs
@@ -1,4 +1,5 @@
 use crate::exid::ExId;
+use crate::marks::{ExpandMark, Mark};
 use crate::{AutomergeError, ChangeHash, ObjType, Prop, ReadDoc, ScalarValue};
 
 /// A way of mutating a document within a single change.
@@ -88,6 +89,21 @@ pub trait Transactable: ReadDoc {
         text: &str,
     ) -> Result<(), AutomergeError>;
 
+    fn mark<O: AsRef<ExId>>(
+        &mut self,
+        obj: O,
+        mark: Mark<'_>,
+        expand: ExpandMark,
+    ) -> Result<(), AutomergeError>;
+
+    fn unmark<O: AsRef<ExId>>(
+        &mut self,
+        obj: O,
+        key: &str,
+        start: usize,
+        end: usize,
+    ) -> Result<(), AutomergeError>;
+
     /// The heads this transaction will be based on
     fn base_heads(&self) -> Vec<ChangeHash>;
 }
diff --git a/rust/automerge/src/types.rs b/rust/automerge/src/types.rs
index 468986ec..468605b6 100644
--- a/rust/automerge/src/types.rs
+++ b/rust/automerge/src/types.rs
@@ -14,6 +14,7 @@ mod opids;
 pub(crate) use opids::OpIds;
 
 pub(crate) use crate::clock::Clock;
+pub(crate) use crate::marks::MarkData;
 pub(crate) use crate::value::{Counter, ScalarValue, Value};
 
 pub(crate) const HEAD: ElemId = ElemId(OpId(0, 0));
@@ -198,6 +199,8 @@ pub enum OpType {
     Delete,
     Increment(i64),
     Put(ScalarValue),
+    MarkBegin(bool, MarkData),
+    MarkEnd(bool),
 }
 
 impl OpType {
@@ -213,6 +216,7 @@ impl OpType {
             Self::Make(ObjType::Text) => 4,
             Self::Increment(_) => 5,
             Self::Make(ObjType::Table) => 6,
+            Self::MarkBegin(_, _) | Self::MarkEnd(_) => 7,
         }
     }
 
@@ -227,11 +231,17 @@ impl OpType {
                 _ => Err(error::InvalidOpType::NonNumericInc),
             },
             6 => Ok(()),
+            7 => Ok(()),
             _ => Err(error::InvalidOpType::UnknownAction(action)),
         }
     }
 
-    pub(crate) fn from_action_and_value(action: u64, value: ScalarValue) -> OpType {
+    pub(crate) fn from_action_and_value(
+        action: u64,
+        value: ScalarValue,
+        mark_name: Option<smol_str::SmolStr>,
+        expand: bool,
+    ) -> OpType {
         match action {
             0 => Self::Make(ObjType::Map),
             1 => Self::Put(value),
@@ -244,9 +254,27 @@ impl OpType {
                 _ => unreachable!("validate_action_and_value returned NonNumericInc"),
             },
             6 => Self::Make(ObjType::Table),
+            7 => match mark_name {
+                Some(name) => Self::MarkBegin(expand, MarkData { name, value }),
+                None => Self::MarkEnd(expand),
+            },
             _ => unreachable!("validate_action_and_value returned UnknownAction"),
         }
     }
+
+    pub(crate) fn to_str(&self) -> &str {
+        if let OpType::Put(ScalarValue::Str(s)) = &self {
+            s
+        } else if self.is_mark() {
+            ""
+        } else {
+            "\u{fffc}"
+        }
+    }
+
+    pub(crate) fn is_mark(&self) -> bool {
+        matches!(&self, OpType::MarkBegin(_, _) | OpType::MarkEnd(_))
+    }
 }
 
 impl From<ObjType> for OpType {
@@ -426,6 +454,13 @@ impl Display for Prop {
 }
 
 impl Key {
+    pub(crate) fn prop_index(&self) -> Option<usize> {
+        match self {
+            Key::Map(n) => Some(*n),
+            Key::Seq(_) => None,
+        }
+    }
+
     pub(crate) fn elemid(&self) -> Option<ElemId> {
         match self {
             Key::Map(_) => None,
@@ -458,6 +493,16 @@ impl OpId {
             .cmp(&other.0)
             .then_with(|| actors[self.1 as usize].cmp(&actors[other.1 as usize]))
     }
+
+    #[inline]
+    pub(crate) fn prev(&self) -> OpId {
+        OpId(self.0 - 1, self.1)
+    }
+
+    #[inline]
+    pub(crate) fn next(&self) -> OpId {
+        OpId(self.0 + 1, self.1)
+    }
 }
 
 #[derive(Debug, Clone, Copy, PartialOrd, Eq, PartialEq, Ord, Hash, Default)]
@@ -582,14 +627,20 @@ impl Op {
     }
 
     pub(crate) fn to_str(&self) -> &str {
-        if let OpType::Put(ScalarValue::Str(s)) = &self.action {
-            s
-        } else {
-            "\u{fffc}"
-        }
+        self.action.to_str()
     }
 
     pub(crate) fn visible(&self) -> bool {
+        if self.is_inc() || self.is_mark() {
+            false
+        } else if self.is_counter() {
+            self.succ.len() <= self.incs()
+        } else {
+            self.succ.is_empty()
+        }
+    }
+
+    pub(crate) fn visible_or_mark(&self) -> bool {
         if self.is_inc() {
             false
         } else if self.is_counter() {
@@ -619,6 +670,18 @@ impl Op {
         matches!(&self.action, OpType::Put(ScalarValue::Counter(_)))
     }
 
+    pub(crate) fn is_mark(&self) -> bool {
+        self.action.is_mark()
+    }
+
+    pub(crate) fn valid_mark_anchor(&self) -> bool {
+        self.succ.is_empty()
+            && matches!(
+                &self.action,
+                OpType::MarkBegin(true, _) | OpType::MarkEnd(false)
+            )
+    }
+
     pub(crate) fn is_noop(&self, action: &OpType) -> bool {
         matches!((&self.action, action), (OpType::Put(n), OpType::Put(m)) if n == m)
     }
@@ -655,6 +718,10 @@ impl Op {
         match &self.action {
             OpType::Make(obj_type) => Value::Object(*obj_type),
             OpType::Put(scalar) => Value::Scalar(Cow::Borrowed(scalar)),
+            OpType::MarkBegin(_, mark) => {
+                Value::Scalar(Cow::Owned(format!("markBegin={}", mark.value).into()))
+            }
+            OpType::MarkEnd(_) => Value::Scalar(Cow::Owned("markEnd".into())),
             _ => panic!("cant convert op into a value - {:?}", self),
         }
     }
@@ -675,6 +742,8 @@ impl Op {
             OpType::Make(obj) => format!("make{}", obj),
             OpType::Increment(val) => format!("inc:{}", val),
             OpType::Delete => "del".to_string(),
+            OpType::MarkBegin(_, _) => "markBegin".to_string(),
+            OpType::MarkEnd(_) => "markEnd".to_string(),
         }
     }
 }
diff --git a/rust/automerge/src/value.rs b/rust/automerge/src/value.rs
index be128787..22a24c2b 100644
--- a/rust/automerge/src/value.rs
+++ b/rust/automerge/src/value.rs
@@ -341,6 +341,12 @@ impl<'a> From<ScalarValue> for Value<'a> {
     }
 }
 
+impl<'a> From<&'a ScalarValue> for Value<'a> {
+    fn from(v: &'a ScalarValue) -> Self {
+        Value::Scalar(Cow::Borrowed(v))
+    }
+}
+
 #[derive(Deserialize, Serialize, PartialEq, Debug, Clone, Copy)]
 pub(crate) enum DataType {
     #[serde(rename = "counter")]
diff --git a/rust/automerge/src/visualisation.rs b/rust/automerge/src/visualisation.rs
index 31e9bbdb..4e639141 100644
--- a/rust/automerge/src/visualisation.rs
+++ b/rust/automerge/src/visualisation.rs
@@ -249,6 +249,8 @@ impl OpTableRow {
             crate::OpType::Put(v) => format!("set {}", v),
             crate::OpType::Make(obj) => format!("make {}", obj),
             crate::OpType::Increment(v) => format!("inc {}", v),
+            crate::OpType::MarkBegin(_, m) => format!("markEnd {}", m),
+            crate::OpType::MarkEnd(m) => format!("markEnd {}", m),
         };
         let prop = match op.key {
             crate::types::Key::Map(k) => metadata.props[k].clone(),
diff --git a/rust/automerge/tests/test.rs b/rust/automerge/tests/test.rs
index 3be6725e..3ba1df40 100644
--- a/rust/automerge/tests/test.rs
+++ b/rust/automerge/tests/test.rs
@@ -6,7 +6,7 @@ use automerge::{
 use std::fs;
 
 // set up logging for all the tests
-//use test_log::test;
+use test_log::test;
 
 #[allow(unused_imports)]
 use automerge_test::{