code: func (t *TxStructure) Clear(key []byte) error {
  	if t.readWriter == nil {
  		return errWriteOnSnapshot
  	}
  	ek := t.encodeStringDataKey(key)
  	err := t.readWriter.Delete(ek)
  	if kv.ErrNotExist.Equal(err) {
  		err = nil
  	}
  	return errors.Trace(err)
  }
docs: // Clear removes the string value of the key.