c -= 'a' - 'A'
			}
			b[i] = c
		}
		return b
	}
	return Map(unicode.ToUpper, s)
}

// ToLower returns a copy of the byte slice s with all Unicode letters mapped to
// their lower case.
func ToLower(s []byte) []byte {
	isASCII, hasUpper := true, false
	for i := 0; i < len(s); i++ {
		c := s[i]
		if c >= utf8.RuneSelf {
			isASCII = false
			break
		}