public NavigableSet<E> descendingSet() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate().descendingSet();
    }

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

    @Override
    public NavigableSet<E> headSet(E toElement, boolean inclusive) {
      assertTrue(Thread.holdsLock(mutex));

    @Override
    @CheckForNull
    public E lower(@ParametricNullness E e) {
      return getElementOrNull(multiset().headMultiset(e, OPEN).lastEntry());
    }

    @Override
    @CheckForNull
    public E floor(@ParametricNullness E e) {
      return getElementOrNull(multiset().headMultiset(e, CLOSED).lastEntry());
    }

    @Override
    @CheckForNull
    public E ceiling(@ParametricNullness E e) {

      return standardSubSet(fromElement, toElement);
    }

    @Override
    public @Nullable T lower(T e) {
      return standardLower(e);
    }

    @Override
    public @Nullable T floor(T e) {
      return standardFloor(e);
    }

    @Override
    public @Nullable T ceiling(T e) {
      return standardCeiling(e);
    }

    @Override
    public @Nullable T higher(T e) {

      return standardSubSet(fromElement, toElement);
    }

    @Override
    public @Nullable T lower(T e) {
      return standardLower(e);
    }

    @Override
    public @Nullable T floor(T e) {
      return standardFloor(e);
    }

    @Override
    public @Nullable T ceiling(T e) {
      return standardCeiling(e);
    }

    @Override
    public @Nullable T higher(T e) {

  @CheckForNull
  public E lower(E element) {
    int index = headIndex(element, false) - 1;
    return (index == -1) ? null : elements.get(index);
  }

  @Override
  @CheckForNull
  public E floor(E element) {
    int index = headIndex(element, true) - 1;
    return (index == -1) ? null : elements.get(index);
  }

  @Override
  @CheckForNull
  public E ceiling(E element) {