@Override
    public E removeLast() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.removeLast();
    }

    @Override
    public @Nullable E pollFirst() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.pollFirst();
    }

    @Override
    public @Nullable E pollLast() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.pollLast();
    }

        control.add(element);
        q.add(element);
      } else {
        assertEqualsUsingSeed(seed, control.poll(), q.pollFirst());
      }
    }
    while (!control.isEmpty()) {
      assertEqualsUsingSeed(seed, control.poll(), q.pollFirst());
    }
    assertTrue(q.isEmpty());
  }

  public void testExhaustive_pollAndPush() {
    int size = 5;

    @Override
    public E removeLast() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.removeLast();
    }

    @Override
    public @Nullable E pollFirst() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.pollFirst();
    }

    @Override
    public @Nullable E pollLast() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.pollLast();
    }

    }

    @Override
    public Iterator<E> descendingIterator() {
      return descendingSet().iterator();
    }

    @Override
    public @Nullable E pollFirst() {
      return getElementOrNull(multiset().pollFirstEntry());
    }

    @Override
    public @Nullable E pollLast() {
      return getElementOrNull(multiset().pollLastEntry());
    }

    public @Nullable E higher(@ParametricNullness E e) {
      return forward.lower(e);
    }

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

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

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

    @Override

    }

    @Override
    public @Nullable V higher(@ParametricNullness V v) {
      return getSortedSetDelegate().higher(v);
    }

    @Override
    public @Nullable V pollFirst() {
      return Iterators.pollNext(iterator());
    }

    @Override
    public @Nullable V pollLast() {
      return Iterators.pollNext(descendingIterator());
    }

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

   */
  @CanIgnoreReturnValue
  @Deprecated
  @GwtIncompatible // NavigableSet
  @Override
  @DoNotCall("Always throws UnsupportedOperationException")
  public final @Nullable E pollFirst() {
    throw new UnsupportedOperationException();
  }

  /**
   * Guaranteed to throw an exception and leave the set unmodified.
   *
   * @since 12.0
   * @throws UnsupportedOperationException always