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_fails_if_key_function_is_null() {
    assertThatThrownBy(() -> unorderedIndex(null))
      .isInstanceOf(NullPointerException.class)
      .hasMessage("Key function can't be null");
  }