func NewUniqueStringsValue(s string) (us *UniqueStringsValue) {
	us = &UniqueStringsValue{Values: make(map[string]struct{})}
	if s == "" {
		return us
	}
	if err := us.Set(s); err != nil {
		plog.Panicf("new UniqueStringsValue should never fail: %v", err)
	}
	return us
}
func NewUniqueStringsValue(s string) (us *UniqueStringsValue) {
	us = &UniqueStringsValue{Values: make(map[string]struct{})}
	if s == "" {
		return us
	}
	if err := us.Set(s); err != nil {
		plog.Panicf("new UniqueStringsValue should never fail: %v", err)
	}
	return us
}
// NewUniqueStringsValue implements string slice as "flag.Value" interface.
// Given value is to be separated by comma.
// The values are set in order.