code: func StringToSerial(serial string) (*big.Int, error) {
  	var serialNum big.Int
  	if !ValidSerial(serial) {
  		return &serialNum, errors.New("Invalid serial number")
  	}
  	_, err := fmt.Sscanf(serial, "%036x", &serialNum)
  	return &serialNum, err
  }
docs: // StringToSerial converts a string into a certificate serial number (big.Int)
  // consistently.