anchor.diff:            oldData = data;
             return Future.forEach(data,
                 (item) => collection.update({'_id': item['_id']},
  -                  prepare(modifier(item)))
  +                  prepare(item))
                 );
           });
         }).then((_) {
  
positive.code:         num versionUpdate = nextVersion;
  
          Map prepare(Map document) {
            document[VERSION_FIELD_NAME] = versionUpdate++;
            return document;
          }
  
negative.code:         }
  
          if (!newData.isEmpty && newData['_id'] != _id) {
            throw new MongoException(null,null,
                'New document id ${newData['_id']} should be same as old one $_id.');
          } else {
            return _maxVersion.then((version) {
  