anchor.diff:              }));
           }
         }).then((_) => _release_locks()).then((_) => nextVersion)
  -      .catchError((e) => _release_locks().then((_) => _processError(e)));
  +      .catchError((e, s) => _release_locks().then((_) => _processError(e, s)));
     }
   
     Future update(selector, Map modifier(Map document), String author) {
  
positive.code:   }
  
  
    _processError(var e) {
      if (e is BreakException) {
        return e.val;
      } else {
        throw e;
      }
    }
  
negative.code: const String LOCK_COLLECTION_NAME = '__clean_lock';
  final Function historyCollectionName =
    (collectionName) => "__clean_${collectionName}_history";
  
  class MongoDatabase {
    Db _db;
  