assertEquals(a, sortedMultiset.headMultiset(b.getElement(), OPEN).lastEntry());
    assertEquals(a, sortedMultiset.headMultiset(c.getElement(), OPEN).lastEntry());
  }

  @CollectionSize.Require(SEVERAL)
  public void testFloor() {
    resetWithHole();
    assertEquals(a, sortedMultiset.headMultiset(a.getElement(), CLOSED).lastEntry());

    assertEquals(a, sortedMultiset.headMultiset(b.getElement(), OPEN).lastEntry());
    assertEquals(a, sortedMultiset.headMultiset(c.getElement(), OPEN).lastEntry());
  }

  @CollectionSize.Require(SEVERAL)
  public void testFloor() {
    resetWithHole();
    assertEquals(a, sortedMultiset.headMultiset(a.getElement(), CLOSED).lastEntry());

    assertNull(navigableMap.higherEntry(k0()));
    assertNull(navigableMap.higherKey(k0()));
  }

  @CollectionSize.Require(ZERO)
  public void testEmptyMapLast() {
    assertNull(navigableMap.lastEntry());
  }

  @MapFeature.Require(SUPPORTS_REMOVE)
  @CollectionSize.Require(ZERO)
  public void testEmptyMapPollLast() {
    assertNull(navigableMap.pollLastEntry());
  }

    return result;
  }

  @Override
  @CheckForNull
  public Entry<E> firstEntry() {
    return delegate().firstEntry();
  }

  @Override
  @CheckForNull
  public Entry<E> lastEntry() {
    return delegate().lastEntry();
  }

  @Override
  @CheckForNull
  public Entry<E> pollFirstEntry() {
    throw new UnsupportedOperationException();
  }

  @Override
  @CheckForNull

    Entry<Cut<K>, RangeMapEntry<K, V>> lastEntry = entriesByLowerBound.lastEntry();
    // Either both are null or neither is, but we check both to satisfy the nullness checker.
    if (firstEntry == null || lastEntry == null) {
      throw new NoSuchElementException();
    }
    return Range.create(
        firstEntry.getValue().getKey().lowerBound, lastEntry.getValue().getKey().upperBound);
  }

    Entry<Cut<K>, RangeMapEntry<K, V>> lastEntry = entriesByLowerBound.lastEntry();
    // Either both are null or neither is, but we check both to satisfy the nullness checker.
    if (firstEntry == null || lastEntry == null) {
      throw new NoSuchElementException();
    }
    return Range.create(
        firstEntry.getValue().getKey().lowerBound, lastEntry.getValue().getKey().upperBound);
  }

  @CheckForNull
  public Entry<K, V> lowerEntry(K key) {
    return headMap(key, false).lastEntry();
  }

  @Override
  @CheckForNull
  public K lowerKey(K key) {
    return keyOrNull(lowerEntry(key));
  }

  @Override
  @CheckForNull
  public Entry<K, V> floorEntry(K key) {
    return headMap(key, true).lastEntry();
  }

  @Override
  @CheckForNull
  public K floorKey(K key) {

  @CheckForNull
  public Entry<K, V> lowerEntry(K key) {
    return headMap(key, false).lastEntry();
  }

  @Override
  @CheckForNull
  public K lowerKey(K key) {
    return keyOrNull(lowerEntry(key));
  }

  @Override
  @CheckForNull
  public Entry<K, V> floorEntry(K key) {
    return headMap(key, true).lastEntry();
  }

  @Override
  @CheckForNull
  public K floorKey(K key) {

      synchronized (mutex) {
        return delegate().higherKey(key);
      }
    }

    @Override
    @CheckForNull
    public Map.Entry<K, V> lastEntry() {
      synchronized (mutex) {
        return nullableSynchronizedEntry(delegate().lastEntry(), mutex);
      }
    }

    @Override
    @CheckForNull
    public Map.Entry<K, V> lowerEntry(K key) {
      synchronized (mutex) {

      synchronized (mutex) {
        return delegate().higherKey(key);
      }
    }

    @Override
    @CheckForNull
    public Map.Entry<K, V> lastEntry() {
      synchronized (mutex) {
        return nullableSynchronizedEntry(delegate().lastEntry(), mutex);
      }
    }

    @Override
    @CheckForNull
    public Map.Entry<K, V> lowerEntry(K key) {
      synchronized (mutex) {