public static ReduceByKey.CombineFunctionWithIdentity<Double> ofDoubles() {
    return SUMS_OF_DOUBLE;
  }
@Test
  public void testSumOfDoubles() {
    assertEquals(6.0, (double) apply(Stream.of(1.0, 2.0, 3.0), Sums.ofDoubles()), 0.001);
  }