diff --git a/client/index.ts b/client/index.ts
index fbd3f98..582ffe4 100644
--- a/client/index.ts
+++ b/client/index.ts
@@ -1,3 +1,4 @@
+export * from './lib/source/mock-source'
 export * from './lib/source/poll-source'
 export * from './lib/source/ws-source'
 export * from './lib/source/source'
diff --git a/client/lib/json-patch-lib.ts b/client/lib/json-patch-lib.ts
index bd60cf1..a961aa0 100644
--- a/client/lib/json-patch-lib.ts
+++ b/client/lib/json-patch-lib.ts
@@ -1,3 +1,5 @@
+import { PatchOp } from './types'
+
 export interface Validator<T> {
   (operation: Operation, index: number, doc: T, existingPathFragment: string): void
 }
@@ -7,16 +9,16 @@ export interface BaseOperation {
 }
 
 export interface AddOperation<T> extends BaseOperation {
-  op: 'add'
+  op: PatchOp.ADD
   value: T
 }
 
 export interface RemoveOperation extends BaseOperation {
-  op: 'remove'
+  op: PatchOp.REMOVE
 }
 
 export interface ReplaceOperation<T> extends BaseOperation {
-  op: 'replace'
+  op: PatchOp.REPLACE
   value: T
 }
 
@@ -55,13 +57,13 @@ export function applyOperation (doc: Doc, op: Operation): Operation | null {
       node[key] = op.value
       if (curVal) {
         undo = {
-          op: 'replace',
+          op: PatchOp.REPLACE,
           path: op.path,
           value: curVal,
         }
       } else {
         undo = {
-          op: 'remove',
+          op: PatchOp.REMOVE,
           path: op.path,
         }
       }
@@ -69,7 +71,7 @@ export function applyOperation (doc: Doc, op: Operation): Operation | null {
       delete node[key]
       if (curVal) {
         undo = {
-          op: 'add',
+          op: PatchOp.ADD,
           path: op.path,
           value: curVal,
         }
diff --git a/client/lib/source/mock-source.ts b/client/lib/source/mock-source.ts
new file mode 100644
index 0000000..f67398f
--- /dev/null
+++ b/client/lib/source/mock-source.ts
@@ -0,0 +1,14 @@
+import { Observable } from 'rxjs'
+import { Update } from '../types'
+import { Source } from './source'
+
+export class MockSource<T> implements Source<T> {
+
+  constructor (
+    private readonly seed: Observable<Update<T>>,
+  ) { }
+
+  watch$ (): Observable<Update<T>> {
+    return this.seed
+  }
+}