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create a hash to hold processed pairs, with a key like "3 4" to 
indicate that 3 has been processed against 4
...
            if not
                 process the pair
                 mark the pair in the hash

minimum = 3, maximum = 11, so loop from 3 to 7
first interval is 3-7, so includes 3,4,6,7
...
  skip 7 & 8, already done
  process 7 & 11
  process 8 & 11