assertFalse(powerSet.contains(Collections.<@Nullable Integer>singleton(null)));
    assertFalse(powerSet.contains(null));
    assertFalse(powerSet.contains((Object) "notASet"));
  }

  public void testPowerSetIteration_manual() {
    ImmutableSet<Integer> elements = ImmutableSet.of(1, 2, 3);
    Set<Set<Integer>> powerSet = powerSet(elements);
    // The API doesn't promise this iteration order, but it's convenient here.