func ToTitleSpecial(c unicode.SpecialCase, s string) string {
	return Map(c.ToTitle, s)
}

// ToValidUTF8 returns a copy of the string s with each run of invalid UTF-8 byte sequences
// replaced by the replacement string, which may be empty.
func ToValidUTF8(s, replacement string) string {
	var b Builder

	for i, c := range s {
		if c != utf8.RuneError {
			continue
		}