code: func ValidateProgressOutput(val string) error {
  	valid := []string{"auto", "plain", "tty"}
  	for _, s := range valid {
  		if s == val {
  			return nil
  		}
  	}
  	return fmt.Errorf("invalid value %q passed to --progress, valid values are: %s", val, strings.Join(valid, ", "))
  }
docs: // ValidateProgressOutput errors out if an invalid value is passed to --progress