public void testPollFirst() {
    assertEquals(a, navigableSet.pollFirst());
    assertEquals(values.subList(1, values.size()), copyToList(navigableSet));
  }

  @CollectionFeature.Require(absent = SUPPORTS_REMOVE)
  public void testPollFirstUnsupported() {
    assertThrows(UnsupportedOperationException.class, () -> navigableSet.pollFirst());
  }

  @CollectionSize.Require(SEVERAL)
  public void testLowerHole() {

    @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();
    }

    return delegate.last();
  }

  @Override
  public @Nullable E lower(E e) {
    return delegate.lower(checkValid(e));
  }

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

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

  @Override
  public boolean remove(Object object) {

  public void testPollFirst() {
    assertEquals(a, navigableSet.pollFirst());
    assertEquals(values.subList(1, values.size()), copyToList(navigableSet));
  }

  @CollectionFeature.Require(absent = SUPPORTS_REMOVE)
  public void testPollFirstUnsupported() {
    assertThrows(UnsupportedOperationException.class, () -> navigableSet.pollFirst());
  }

  @CollectionSize.Require(SEVERAL)
  public void testLowerHole() {

  }

  @Override
  public E takeLast() throws InterruptedException {
    return delegate().takeLast();
  }

  @Override
  @CheckForNull
  public E pollFirst(long timeout, TimeUnit unit) throws InterruptedException {
    return delegate().pollFirst(timeout, unit);
  }

  @Override
  @CheckForNull
  public E pollLast(long timeout, TimeUnit unit) throws InterruptedException {

  }

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

  /**
   * A sensible definition of {@link #pollFirst} in terms of the {@code iterator} method. If you
   * override {@link #iterator} you may wish to override {@link #pollFirst} to forward to this
   * implementation.
   */
  @CheckForNull
  protected E standardPollFirst() {

  @CheckForNull
  public E peekLast() {
    return delegate().peekLast();
  }

  @CanIgnoreReturnValue // TODO(cpovirk): Consider removing this?
  @Override
  @CheckForNull
  public E pollFirst() {
    return delegate().pollFirst();
  }

  @CanIgnoreReturnValue // TODO(cpovirk): Consider removing this?
  @Override
  @CheckForNull
  public E pollLast() {
    return delegate().pollLast();
  }

  }

  @Override
  public E takeLast() throws InterruptedException {
    return delegate().takeLast();
  }

  @Override
  @CheckForNull
  public E pollFirst(long timeout, TimeUnit unit) throws InterruptedException {
    return delegate().pollFirst(timeout, unit);
  }

  @Override
  @CheckForNull
  public E pollLast(long timeout, TimeUnit unit) throws InterruptedException {

  }

  @Override
  public E takeLast() throws InterruptedException {
    return delegate().takeLast();
  }

  @Override
  @CheckForNull
  public E pollFirst(long timeout, TimeUnit unit) throws InterruptedException {
    return delegate().pollFirst(timeout, unit);
  }

  @Override
  @CheckForNull
  public E pollLast(long timeout, TimeUnit unit) throws InterruptedException {

    }

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

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

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