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Proper Luhn AddCheckDigit for any length #

By Bruce D. Rempe,

Most of the luhn scripts assume that you are working with credit-cards and then IMPROPERLY calculate their luhn check-digit by starting at the left side (assuming that the core number is always 15 digits).  However, if you don't know the length of your core number, then such an approach will fail when the length is even.  This function will work without regard to the length of the core number because it properly starts it's calculations from the right side of the number.

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