private abstract static class EmptyIteratorTester extends IteratorTester<Integer> {
    EmptyIteratorTester() {
      super(3, MODIFIABLE, Collections.<Integer>emptySet(), IteratorTester.KnownOrder.KNOWN_ORDER);
    }
  }

  @GwtIncompatible // Only used by @GwtIncompatible code
  private abstract static class SingletonIteratorTester extends IteratorTester<Integer> {
    SingletonIteratorTester() {