anchor.diff:      }
       return selector;
     }
  -}+}
  
positive.code:   String collectionName;
    DbCollection(this.db, this.collectionName){}
    String fullName() => "${db.databaseName}.$collectionName";
    void save(Map document, {WriteConcern writeConcern}){
      var id;
      bool createId = false;
      if (document.containsKey("_id")){
  
negative.code:       }
      }
      if (id != null){
        return update({"_id": id}, document, writeConcern: writeConcern);
      }
      else{
        if (createId) {
          document["_id"] = new ObjectId();
        }
        return insert(document, writeConcern: writeConcern);
      }
    }
   Future insertAll(List<Map> documents, {WriteConcern writeConcern}){
  