code: func (yn *YesOrNo) Set(v string) error {
  	switch v {
  	case "yes":
  		*yn = true
  	case "no":
  		*yn = false
  	default:
  		return errors.New("expecting 'yes' or 'no'")
  	}
  	return nil
  }
docs: // Set changes the value of YesOrNo.