anchor.diff:     * Adds document [data] to database. If document with same [_id] alreay
      * exists, nothing happens and [true] is returned.
      */
  -  Future add(Map data, String author) {
  +  Future __add(Map data, String author) {
       cache.invalidate();
       num nextVersion;
       return _get_locks().then((_) =>
  
positive.code:       ).then((_) => _release_locks()).then((_) => nextVersion);
    }
  
    /**
     * Changes document with id [_id] to [newData]. If such document does not
     * exist, nothing happens and [true] is returned.
     */
    Future change(String _id, Map newData, String author) {
      cache.invalidate();
      num nextVersion;
      Map newRecord;
  
negative.code:               return _limitedDiffFromVersion(diff);
              }
  
              return _prettify(diff);
      }).catchError((e){
       if (e is List) {
         return e;
  