anchor.diff:        return new BsonArray([]);
       case _BSON_DATA_OBJECT:
         return new BsonMap({});
  +    case _BSON_DATA_UNDEFINED:
  +      return new BsonNull();
       case _BSON_DATA_OID:
         return new ObjectId();
       case _BSON_DATA_NULL:
  
positive.code: const _BSON_DATA_CODE = 13;
  
  /** Timestamp BSON Type **/
  const _BSON_DATA_TIMESTAMP = 17; 
  
  /** The following types are implemented partially **/
  //static const BSON_DATA_MIN_KEY = 0xff; MinKey BSON Type
  
negative.code: 
  /** BsonBinary BSON Type **/
  const _BSON_DATA_BINARY = 5;
  
  /** undefined BSON Type **/
  const _BSON_DATA_UNDEFINED = 6;
  
  /** ObjectID BSON Type **/
  const _BSON_DATA_OID = 7;
  