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>() {

    assertEquals(ImmutableSet.of(1, 2, 3), sync.inverse().keySet());
  }

  private static final Function<Integer, Double> SQRT_FUNCTION = in -> Math.sqrt(in);

  public void testTransformValues() {
    Map<String, Integer> map = ImmutableMap.of("a", 4, "b", 9);
    Map<String, Double> transformed = transformValues(map, SQRT_FUNCTION);

    assertEquals(ImmutableMap.of("a", 2.0, "b", 3.0), transformed);