def checksum
      evens, odds = even_and_odd_digits
      sum = odds.inject(0){|s,d| s + d } + evens.inject(0){|s,d| s + (d*3) }
      sum %= 10
      sum.zero? ? 0 : 10-sum
    end
def checksum
      evens, odds = even_and_odd_digits
      sum = odds.inject(0){|s,d| s + d } + evens.inject(0){|s,d| s + (d*3) }
      sum %= 10
      sum.zero? ? 0 : 10-sum
    end
Mod10