public static Combine.CombineFn<Boolean, ?, Long> combineFn() {
    return new CountIfFn();
  }
@Test
  public void testMergeAccumulators() {
    Combine.CombineFn countIfFn = CountIf.combineFn();
    List<long[]> accums = Arrays.asList(new long[] {2}, new long[] {2});
    long[] accumulator = (long[]) countIfFn.mergeAccumulators(accums);

    assertEquals(4L, accumulator[0]);
  }