FMTCC(SEXP) (STRING)

 Function
  Formats a credit card number for display purposes.

 Syntax
  FMTCC(sexp)

   sexp = Any string expression.

 Return Type & Value
  STRING
  Returns sexp formatted in a credit card style for display purposes.

 Remarks
  This function will do one of the following:  one, take a 13 character
  string and format it as "XXXX XXX XXX XXX"; two, take a 15 character
  string and format it as "XXXX XXXXXX XXXXX"; three, take a 16 character
  string and format it as "XXXX XXXX XXXX XXXX"; or four, return the
  string unmodified if it is not 13, 15 or 16 characters long.

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