Version 1.00
  03/25/02 - Fixed linguistic problem in error message ("not correct" vs. 
    "incorrect").

Version 0.13

  05/03/01 - Changed is_valid to always return 1 or '' (instead of 0 or
    sometimes undef) and it sets $ERROR when the check_digit doesn't match.

  05/03/01 - Small change to the calculation of the return value of check_digit.
    Purely for aesthetic reasons.

Version 0.12

  05/02/01 - Replaced the ill-conceived C<die> call in check_digit with the
    $ERROR variable that contains the reason for the "failure."