public static <K, E> Collector<E, ImmutableListMultimap.Builder<K, E>, ImmutableListMultimap<K, E>> index(Function<? super E, K> keyFunction) {
    return index(keyFunction, Function.identity());
  }
@Test
  public void index_parallel_stream() {
    ListMultimap<String, String> multimap = HUGE_LIST.parallelStream().collect(index(identity()));

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