NavigableSet<E> outer = Synchronized.navigableSet(inner, null);
    inner.mutex = outer;
    return outer;
  }

  static class TestSet<E> extends SynchronizedSetTest.TestSet<E> implements NavigableSet<E> {

    TestSet(NavigableSet<E> delegate, @Nullable Object mutex) {
      super(delegate, mutex);
    }

    @Override
    protected NavigableSet<E> delegate() {

      return super.equals(o);
    }

    @Override
    public int hashCode() {
      assertTrue(Thread.holdsLock(mutex));
      return super.hashCode();
    }

    @Override
    public int size() {
      assertTrue(Thread.holdsLock(mutex));
      return super.size();
    }

    @Override
    public boolean isEmpty() {
      assertTrue(Thread.holdsLock(mutex));