def _checksum
          sum = 0
          10.step( 2, -1 ) {
               |n|
               m = 10 - n
               sum += n * @isbn[m..m].to_i
               }
          checksum = ( 11 - ( sum % 11 ) ) % 11
          checksum = 'X' if checksum == 10
          return checksum.to_s
     end

Example 4: Ruby version of the _checksum routine shown in Example 3.