public static <NumT extends Number>
      Combine.AccumulatingCombineFn<NumT, CountSum<NumT>, Double> of() {
    return new MeanFn<>();
  }
@Test
  public void testMeanFn() throws Exception {
    testCombineFn(Mean.of(), Lists.newArrayList(1, 2, 3, 4), 2.5);
  }