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