These subroutines tell you whether a credit card number is self-consistent
-- whether the last digit of the number is a valid checksum for the
preceding digits.

The validate() subroutine returns 1 if the card number provided passes the
checksum test, and 0 otherwise.

The cardtype() subroutine returns a string containing the type of card. The
list of possible return values is more comprehensive than it used to be,
but additions are still most welcome.