VALCC(SEXP) (BOOLEAN)

 Function
  Tests a string for credit card number format validity.

 Syntax
  VALCC(sexp)

   sexp = Any string expression.

 Return Type & Value
  BOOLEAN
  Returns TRUE if the string is a valid credit card number format,
  FALSE otherwise.

 Remarks
  This function will take a string and attempt to identify it as a credit
  card number.  If the number is invalid for any reason (insufficient
  digits or bad checksum, primarily) then this function will return FALSE,
  otherwise it will return TRUE.

 Examples
  STRING s
  WHILE (!VALCC(s)) DO
    INPUT "CC #",s
    NEWLINES 2
  ENDWHILE
  PRINTLN CCTYPE(s)," - ",FMTCC(s)