anchor.diff:    var version = subscription._version;
   
     diff.forEach((Map change) {
  +    // it can happen, that we get too old changes
       if (!change.containsKey('version')){
         logger.warning('change does not contain "version" field. If not testing, '
                        'this is probably bug. (change: $change)');
  
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:   target.length = source.length;
    for (num i=0; i<target.length; i++) {
      if (!applyChange(source[i], target[i], author)) {
        target.set(i, source[i], author: author);
      }
    }
  }
  