public static Combine.BinaryCombineDoubleFn ofDoubles() {
    return new SumDoubleFn();
  }
@Test
  public void testSumDoubleFnNan() {
    testCombineFn(Sum.ofDoubles(), Lists.newArrayList(1.0, 2.0, 3.0, Double.NaN), Double.NaN);
  }