code: func ValidSerial(serial string) bool {
  	// Originally, serial numbers were 32 hex characters long. We later increased
  	// them to 36, but we allow the shorter ones because they exist in some
  	// production databases.
  	if len(serial) != 32 && len(serial) != 36 {
  		return false
  	}
  	_, err := hex.DecodeString(serial)
  	if err != nil {
  		return false
  	}
  	return true
  }
docs: // ValidSerial tests whether the input string represents a syntactically
  // valid serial number, i.e., that it is a valid hex string between 32
  // and 36 characters long.