SUBROUTINE CF.LUHN.CHECK(card.no,result)
*
* LUHN check digit calculation for credit card numbers
*
* card.no: card number for checking passed in
*
* result:  0 = Luhn check passed
*          1 = invalid string
*          2 = check fails
*
*    ========================================================================
*
*    Conditions of use:
*    This source code remains the property and copyright of Gulraj Rijhwani 
*    and Courtfields Limited.  Specific licence is granted to copy and use 
*    this program source, in accordance with the terms and conditions laid 
*    out following.  For the purposes of these terms and conditions the term
*    "developer" is defined to be the individual or company employing the
*    author incorporating use of this source.  Any use of this source code is
*    deemed to be full and unreserved acceptance of these stated terms.
*
*    1)  All instances of this source code must carry all attached
*        annotations and comments including, but not limited to, these
*        these conditions of use in their entirety.
*    2)  The developer may not alter the program source in any way, without
*        first obtaining the express written consent of duly authorised
*        company officers of Courtfields Limited, or its successors.
*    3)  This source code may not be included in whole or in part in any
*        proprietary program suite except as a separately compiled module
*        in wholly unmodified form.  Any installed object code must be
*        accompanied by the original source code at all times.
*    4)  In the event of any breach of these terms and conditions the
*        developer shall be liable for an annual usage charge of 200 pounds
*        sterling for every end user terminal or session using or having
*        access to use the resulting executable code.
*    5)  Gulraj Rijhwani and Courtfields Limited make no warranties for
*        the fitness of this source code, and may not be held liable
*        for any failure to fulfil the expectations or requirements of
*        the developer.
*
*    ========================================================================
*
     result = (card.no='') OR (OCONV(card.no,'MCN')#card.no); * Invalid characters
     IF result THEN RETURN
*
     chk.tot = 0
     no.of.digits = LEN(card.no)
     mpx = 1
     FOR dig.ptr=no.of.digits TO 1 STEP -1
      dig.res = mpx * card.no[dig.ptr,1]
      chk.tot += dig.res - 9 * (dig.res>9)
      mpx = REM(mpx,2) + 1
     NEXT dig.ptr
*
     result = 2 * (REM(chk.tot,10)#0); * Invalid card no
*
     RETURN