code: func (enc *Encoding) UnmarshalFlag(value string) error {
  	switch value {
  	case "flickr":
  		*enc = *FlickrEncoding
  	case "ripple":
  		*enc = *RippleEncoding
  	case "bitcoin":
  		*enc = *BitcoinEncoding
  	default:
  		return fmt.Errorf("unknown encoding: %s", value)
  	}
  	return nil
  }
docs: // UnmarshalFlag implements flags.Unmarshaler