pkg/flags/selective_string.go
func (ss *SelectiveStringValue) Set(s string) error {
	if _, ok := ss.valids[s]; ok {
		ss.v = s
		return nil
	}
	return errors.New("invalid value")
}
func (ss *SelectiveStringValue) Set(s string) error {
	if _, ok := ss.valids[s]; ok {
		ss.v = s
		return nil
	}
	return errors.New("invalid value")
}
// Set verifies the argument to be a valid member of the allowed values
// before setting the underlying flag value.