func ParseBytes(b []byte) (UUID, error) {
	var uuid UUID
	switch len(b) {
	case 36: // xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
	case 36 + 9: // urn:uuid:xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
		if !bytes.Equal(bytes.ToLower(b[:9]), []byte("urn:uuid:")) {
			return uuid, fmt.Errorf("invalid urn prefix: %q", b[:9])
		}
		b = b[9:]
	case 36 + 2: // {xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}
		b = b[1:]
	case 32: // xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
		var ok bool
		for i := 0; i < 32; i += 2 {
			uuid[i/2], ok = xtob(b[i], b[i+1])
			if !ok {
				return uuid, errors.New("invalid UUID format")
			}
		}
		return uuid, nil
	default:
		return uuid, fmt.Errorf("invalid UUID length: %d", len(b))
	}
	// s is now at least 36 bytes long
	// it must be of the form  xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
	if b[8] != '-' || b[13] != '-' || b[18] != '-' || b[23] != '-' {
		return uuid, errors.New("invalid UUID format")
	}
	for i, x := range [16]int{
		0, 2, 4, 6,
		9, 11,
		14, 16,
		19, 21,
		24, 26, 28, 30, 32, 34} {
		v, ok := xtob(b[x], b[x+1])
		if !ok {
			return uuid, errors.New("invalid UUID format")
		}
		uuid[i] = v
	}
	return uuid, nil
}
func ParseBytes(b []byte) (UUID, error) {
	var uuid UUID
	switch len(b) {
	case 36: // xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
	case 36 + 9: // urn:uuid:xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
		if !bytes.Equal(bytes.ToLower(b[:9]), []byte("urn:uuid:")) {
			return uuid, fmt.Errorf("invalid urn prefix: %q", b[:9])
		}
		b = b[9:]
	case 36 + 2: // {xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}
		b = b[1:]
	case 32: // xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
		var ok bool
		for i := 0; i < 32; i += 2 {
			uuid[i/2], ok = xtob(b[i], b[i+1])
			if !ok {
				return uuid, errors.New("invalid UUID format")
			}
		}
		return uuid, nil
	default:
		return uuid, fmt.Errorf("invalid UUID length: %d", len(b))
	}
	// s is now at least 36 bytes long
	// it must be of the form  xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
	if b[8] != '-' || b[13] != '-' || b[18] != '-' || b[23] != '-' {
		return uuid, errors.New("invalid UUID format")
	}
	for i, x := range [16]int{
		0, 2, 4, 6,
		9, 11,
		14, 16,
		19, 21,
		24, 26, 28, 30, 32, 34} {
		v, ok := xtob(b[x], b[x+1])
		if !ok {
			return uuid, errors.New("invalid UUID format")
		}
		uuid[i] = v
	}
	return uuid, nil
}
// ParseBytes is like Parse, except it parses a byte slice instead of a string.