code: func (b *lowBuilder) kvFileType() string {
  	switch {
  	case b.high.SQLite != "":
  		return "sqlite"
  	case b.high.KVFile != "":
  		return "kv"
  	case b.high.LevelDB != "":
  		return "leveldb"
  	default:
  		return sorted.DefaultKVFileType
  	}
  }
docs: // kvFileType returns the file based sorted type defined for index storage, if
  // any. It defaults to "leveldb" otherwise.