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:             }));
          }
        }).then((_) => _release_locks()).then((_) => nextVersion)
        .catchError((e) => _release_locks().then((_) => _processError(e)));
    }
  
    Future change(String _id, Map newData, String author, {clientVersion: null, upsert: false}) {
  
negative.code: const String SET = "\$set";
  const String UNSET = "\$unset";
  const String PUSH = "\$push";
  const String NE = '\$ne';
  const String IN = '\$in';
  
  const num ASC = 1;
  const num DESC = -1;
  const num NOLIMIT = 0;
  