code: func (fkvs *FileKeyValueStore) Delete(key interface{}) error {
  	if key == nil {
  		return errors.New("key is nil")
  	}
  	file, err := fkvs.keySerializer(key)
  	if err != nil {
  		return err
  	}
  	_, err = os.Stat(file)
  	if err != nil {
  		if !os.IsNotExist(err) {
  			return errors.Wrapf(err, "stat dir failed")
  		}
  		// Doesn't exist, OK
  		return nil
  	}
  	return os.Remove(file)
  }
docs: // Delete deletes the value for a key.