anchor.diff:    target.length = source.length;
     for (num i=0; i<target.length; i++) {
       if (!applyChange(source[i], target[i], author)) {
  -      // TODO add set method to DataList and use it here
  -      target.setAll(i, [source[i]], author: author);
  +      target.set(i, source[i], author: author);
       }
     }
   }
  
positive.code:     }
  
      _subscriptions.add(collection.onChangeSync.listen((event) {
        if (event["author"] == null) {
          event["change"].addedItems.forEach((data) {
            Future result = _connection.send(() => new ClientRequest("sync", {
              "action" : "add",
  
negative.code:     if (change["action"] == "add" && change["author"] != author) {
        DataMap record = collection.firstWhere((d) => d["_id"] == change["_id"], orElse : () => null);
        if (record == null) {
          collection.add(new DataMap.from(change["data"]), author: 'clean_sync');
        }
      }
      else if (change["action"] == "change" && change["author"] != author) {
        DataMap record = collection.firstWhere((d) => d["_id"] == change["_id"], orElse : () => null);
        // 1. the change may be for item that is currently not present in the collection;
        // 2. the field may be 'locked', because it was changed on user's machine, and
        // this change was not yet confirmed from server
        if (record != null && subscription._modifiedItems.containsKey(record['_id'])) {
          logger.fine('discarding diff');
        }
         if (record != null && !subscription._modifiedItems.containsKey(record['_id'])) {
          applyChange(change["data"], record, 'clean_sync');
        }
      }
      else if (change["action"] == "remove" && change["author"] != author) {
        collection.removeWhere((d) => d["_id"] == change["_id"], author: 'clean_sync');
      }
      logger.finest('applying finished: $subscription ${subscription.collection} ${subscription._version}');
    });
  