type flagVar []string

func (f *flagVar) String() string {
	return fmt.Sprint([]string(*f))
}

func (f *flagVar) Set(value string) error {
	*f = append(*f, value)
	return nil
}

func TestUserDefined(t *testing.T) {
	var flags FlagSet
	flags.Init("test", ContinueOnError)
	flags.SetOutput(io.Discard)
	var v flagVar
	flags.Var(&v, "v", "usage")