public E lower(E e) {
      synchronized (mutex) {
        return delegate().lower(e);
      }
    }

    @Override
    @CheckForNull
    public E pollFirst() {
      synchronized (mutex) {
        return delegate().pollFirst();
      }
    }

    @Override
    @CheckForNull
    public E pollLast() {
      synchronized (mutex) {
        return delegate().pollLast();

      return forward.lower(e);
    }

    @Override
    @CheckForNull
    public E pollFirst() {
      return forward.pollLast();
    }

    @Override
    @CheckForNull
    public E pollLast() {
      return forward.pollFirst();
    }

    @Override
    public NavigableSet<E> descendingSet() {
      return forward;
    }

    }
    try {
      unmod.addAll(Collections.singleton(4));
      fail("UnsupportedOperationException expected");
    } catch (UnsupportedOperationException expected) {
    }
    try {
      unmod.pollFirst();
      fail("UnsupportedOperationException expected");
    } catch (UnsupportedOperationException expected) {
    }
    try {
      unmod.pollLast();
      fail("UnsupportedOperationException expected");