code: func (opts *ListOpts) Set(value string) error {
  	if opts.validator != nil {
  		v, err := opts.validator(value)
  		if err != nil {
  			return err
  		}
  		value = v
  	}
  	*opts.values = append(*opts.values, value)
  	return nil
  }
docs: // Set validates if needed the input value and adds it to the
  // internal slice.