public E pollFirst() {
      synchronized (mutex) {
        return delegate().pollFirst();
      }
    }

    @Override
    @CheckForNull
    public E pollLast() {
      synchronized (mutex) {
        return delegate().pollLast();
      }
    }

    @Override
    public NavigableSet<E> subSet(
        E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {

    public E pollFirst() {
      synchronized (mutex) {
        return delegate().pollFirst();
      }
    }

    @Override
    @CheckForNull
    public E pollLast() {
      synchronized (mutex) {
        return delegate().pollLast();
      }
    }

    @Override
    public NavigableSet<E> subSet(
        E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {

      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;
    }