anchor.diff:  const String LOCK_COLLECTION_NAME = '__clean_lock';
   final Function historyCollectionName =
     (collectionName) => "__clean_${collectionName}_history";
  -
  -processMongoException(var e){
  -
  -}
   
   class MongoDatabase {
     Db _db;
  
positive.code:             });
              return _prettify(diff);
  
      }).catchError((e) => _processError(e));
    }
  
    num _defaultCompare(a, b) {
  
negative.code:             }));
          }
        }).then((_) => _release_locks()).then((_) => nextVersion)
        .catchError((e, s) => _release_locks().then((_) => _processError(e, s)));
    }
  
    Future update(selector, Map modifier(Map document), String author) {
  