code: func (t *TxStructure) HKeys(key []byte) ([][]byte, error) {
  	var keys [][]byte
  	err := t.iterateHash(key, func(field []byte, value []byte) error {
  		keys = append(keys, append([]byte{}, field...))
  		return nil
  	})
  
  	return keys, errors.Trace(err)
  }
docs: // HKeys gets all the fields in a hash.