func (uuid UUID) URN() string {
	var buf [36 + 9]byte
	copy(buf[:], "urn:uuid:")
	encodeHex(buf[9:], uuid)
	return string(buf[:])
}
func (uuid UUID) URN() string {
	var buf [36 + 9]byte
	copy(buf[:], "urn:uuid:")
	encodeHex(buf[9:], uuid)
	return string(buf[:])
}
// URN returns the RFC 2141 URN form of uuid,
// urn:uuid:xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx,  or "" if uuid is invalid.