ISBN:
 a)1, a)2, ... , a)10			  _
 a)1 + 2*a)2 + 3a)3 + ... + 9a)9 + 10a)10 = 0 (mod 11)
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UPC:
 12 digits - x-xxxxx-xxxxx-x					  _
 3 * (sum of odd position digits) + (sum of even position digits) = 0 (mod 10)
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"Bone" multiplication
  4   3  2
  __________
1 |0/|0/|0/| 2
  |/8|/6|/4|
  |ŻŻŻŻŻŻŻŻ|
0 |2/|1/|1/| 5    432 * 25 = 10800
  |/0|/5|/0|
  ŻŻŻŻŻŻŻŻŻŻ
   8  0  0