List<Integer> values = Arrays.asList(1, 2);
    assertThrows(
        NullPointerException.class, () -> Multimaps.index(values, Functions.constant(null)));
  }

  @GwtIncompatible // untested
  public void testTransformValues() {
    SetMultimap<String, Integer> multimap =
        ImmutableSetMultimap.of("a", 2, "b", -3, "b", 3, "a", 4, "c", 6);
    Function<Integer, Integer> square =
        new Function<Integer, Integer>() {