code: public static String toCreditcard(String strCreditCardNumber) throws ExpressionException {
  	long number = toLong(strCreditCardNumber, -1);
  	if (number == -1) throw new ExpressionException("invalid creditcard number [" + strCreditCardNumber + "]");
  	return toPrettyString(number);
      }
docs: 1800 15 mod 10