public static <K, E> Collector<E, ImmutableSetMultimap.Builder<K, E>, ImmutableSetMultimap<K, E>> unorderedIndex(Function<? super E, K> keyFunction) {
    return unorderedIndex(keyFunction, Function.identity());
  }
@Test
  public void unorderedIndex_parallel_stream() {
    SetMultimap<String, String> multimap = HUGE_LIST.parallelStream().collect(unorderedIndex(identity()));

    assertThat(multimap.keySet()).isEqualTo(HUGE_SET);
  }