draft of YWeakLink type

src/utils/StructStore.js

@@ -2,7 +2,7 @@
 import {
   GC,
   splitItem,
-  Transaction, ID, Item, DSDecoderV2 // eslint-disable-line
+  Transaction, ID, Item, DSDecoderV2, YWeakLink // eslint-disable-line
 } from '../internals.js'
 
 import * as math from 'lib0/math'
@@ -14,6 +14,13 @@ export class StructStore {
      * @type {Map<number,Array<GC|Item>>}
      */
     this.clients = new Map()
+    /**
+     * If this item was referenced by other weak links, here we keep the references
+     * to these weak refs.
+     *
+     * @type {Map<Item, Set<YWeakLink<any>>>}
+     */
+    this.linkedBy = new Map()
     /**
      * @type {null | { missing: Map<number, number>, update: Uint8Array }}
      */