code: func (d *PositiveDurationOpt) Set(s string) error {
  	err := d.DurationOpt.Set(s)
  	if err != nil {
  		return err
  	}
  	if *d.DurationOpt.value < 0 {
  		return errors.Errorf("duration cannot be negative")
  	}
  	return nil
  }
docs: // Set a new value on the option. Setting a negative duration value will cause
  // an error to be returned.