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_with_valueFunction_fails_if_value_function_is_null() {
    assertThatThrownBy(() ->  unorderedIndex(MyObj::getId, null))
      .isInstanceOf(NullPointerException.class)
      .hasMessage("Value function can't be null");
  }