  @sum[$a][$b] += $x;
  @count[$a][$b] += 1;
  end {
    for ((k1, k2), v in @sum) {
      @mean[k1][k2] = @sum[k1][k2] / @count[k1][k2]
    }
    emitp @sum, "a", "b";
    emitp @count, "a", "b";
    emitp @mean, "a", "b"
  }

