code: func (f *Flag) Set(val string) error {
  	if val == "" {
  		return fmt.Errorf("must have an argument value")
  	}
  
  	if f.Data == nil {
  		f.Data = stringset.NewFromSlice(val)
  	} else {
  		f.Data.Add(val)
  	}
  	return nil
  }
docs: // Set implements flag.Value's Set function.