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=head2 KID_ok

Checks if a norwegian KID number has an ok control digit.

To check if a customer has typed the number correctly.

This uses the  LUHN algorithm (also known as mod-10) from 1960 which is also used
internationally in control digits for credit card numbers, and Canadian social security ID numbers as well.

The algorithm, as described in Phrack (47-8) (a long time hacker online publication):

 "For a card with an even number of digits, double every odd numbered
 digit and subtract 9 if the product is greater than 9. Add up all the
 even digits as well as the doubled-odd digits, and the result must be
 a multiple of 10 or it's not a valid card. If the card has an odd
 number of digits, perform the same addition doubling the even numbered
 digits instead."

B<Input:> A KID-nummer. Must consist of digits 0-9 only, otherwise a die (croak) happens.