code: func validatePercent(v string) error {
  	if v == "" {
  		return nil
  	}
  	i, err := strconv.Atoi(v)
  	if err != nil {
  		return fmt.Errorf("invalid integer %q - %s", v, err)
  	}
  	if i < 0 || i > 100 {
  		return fmt.Errorf("%d is out of range (0-100)", i)
  	}
  	return nil
  }
docs: // validatePercent validates a string is empty or an integer between 0-100.