anchor.diff:    }
   
     Future add(Map data, String author, {clientVersion: null}) {
  +    ensureId(data, idgen);
       return writeOperation(data['_id'], author, 'add', data,
           clientVersion: clientVersion);
     }
  
positive.code: 
  MongoProvider mpClone(MongoProvider source){
  
    MongoProvider m = new MongoProvider(source.collection, source._collectionHistory,
        source._lock, source.cache);
    m._selectorList = new List.from(source._selectorList);
    m._sortParams = new Map.from(source._sortParams);
    m._limit = source._limit;
  
negative.code:   num _limit = NOLIMIT;
    num _skip = NOSKIP;
    Cache cache;
    IdGenerator idgen;
  
    //for testing purposes
    Future<int> get maxVersion => _maxVersion;
  