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() {

        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;

  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() {

        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;

        NavigableSet<Integer> filtered = filter(createUnfiltered(contents), EVEN);
        NavigableSet<Integer> unfiltered = createUnfiltered(filtered);

        assertEquals(unfiltered.pollFirst(), filtered.pollFirst());
        assertEquals(unfiltered, filtered);
      }
    }

    public void testPollLast() {
      for (List<Integer> contents : SAMPLE_INPUTS) {

      return standardCeiling(e);
    }

    @Override
    public @Nullable T higher(T e) {
      return standardHigher(e);
    }

    @Override
    public @Nullable T pollFirst() {
      return standardPollFirst();
    }

    @Override
    public @Nullable T pollLast() {
      return standardPollLast();
    }

    @Override

    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();

    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;
    }

    }
  }

  /**
   * Removes and returns the least element of this queue, or returns {@code null} if the queue is
   * empty.
   */
  @CanIgnoreReturnValue
  @CheckForNull
  public E pollFirst() {
    return poll();
  }

  /**
   * Removes and returns the least element of this queue.
   *
   * @throws NoSuchElementException if the queue is empty
   */
  @CanIgnoreReturnValue