@Override
  public AccumT mergeAccumulators(Iterable<AccumT> accumulators) {
    AccumT first = accumulators.iterator().next();
    Iterable<AccumT> rest = new SkipFirstElementIterable<>(accumulators);
    return getAggregateFn().mergeAccumulators(first, rest);
  }
@Test
  public void mergeAccumulators() {
    LazyAggregateCombineFn<Long, Long, Long> combiner = new LazyAggregateCombineFn<>(new Sum());
    long merged = combiner.mergeAccumulators(ImmutableList.of(1L, 1L));
    assertEquals(2L, merged);
  }