my $VAR1 = {
          'A' => [0,1,2],  # centroid is 1, with  other elem 0 and 2 (1 +/- 1)
          'B' => [1,2,3],
          'C' => [2,3,4],
          'D' => [3,4,5],
          'E' => [4,5,6],
          'F' => [5,6],    # centroid is 6 but only with one elem 5 (i.e. 6-1)
          'G' => [8],      # next centroid is 8 but no members
          'I' => [10],     # same for centroid 10
        };

  # As the @nlist array grow larger the final hash will also grow larger.