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

