/* Check a Visa or MC credit card number for accuracy */
Parse Arg CCNumber
Sum=0
Do i=1 to Length(CCNumber)
   if i//2=0 then Do
      Sum=Sum+SubStr(CCNumber,i,1)
      end /* do */
   else Do
      Product=2*SubStr(CCNumber,i,1)
      DigitSum=0
      Do j=1 to Length(Product)
         DigitSum=DigitSum+SubStr(Product,j,1)
      end /* do */
      Sum=Sum+DigitSum
   end /* do */
end /* do */
If Sum//10=0 then 
   Say CCNumber "is valid."
else
   Say CCNumber "is invalid. ("||Sum||")"
Exit