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=head1 USAGE

Using numbers that pass the verhoeff check is useful for things like product codes.
This is because such numbers almost never pass the verhoeff check if they as mis-typed.
This includes common typos such as ommitted or repeated digits, transposed digits and so on.
Since it only adds a single digit onto what might already be a longish number, it's
a good algorithm for use where humans need to enter or read the numbers.

When we say 'number' we really mean 'string of digits' since that is what the Verhoeff
algorithm works on.

To generate such a number, pick a starting number, call verhoeff_check() to get a check digit,
and then APPEND that digit to the end of the original number. Do NOT add the digit arithmetically
to the original number.

The new number will how pass the verhoeff_check(). In other words, if verhoeff_check()
is called with the new number as its argument, it will return 1. If it is called with
a mistyped version of the original number it will (very probably) return 0. For common
forms of typo such as ommitted digits, accuracy is over 99%.