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_empty_stream_returns_empty_map() {
    assertThat(Stream.<MyObj>empty().collect(index(MyObj::getId)).size()).isZero();
    assertThat(Stream.<MyObj>empty().collect(index(MyObj::getId, MyObj::getText)).size()).isZero();
  }