}

    @Override
    public Iterator<E> descendingIterator() {
      return Iterators.filter(unfiltered().descendingIterator(), predicate);
    }

    @Override
    @ParametricNullness
    public E last() {
      return Iterators.find(unfiltered().descendingIterator(), predicate);
    }

    @Override
    public NavigableSet<E> subSet(

    @CheckForNull
    public E ceiling(E e) {
      synchronized (mutex) {
        return delegate().ceiling(e);
      }
    }

    @Override
    public Iterator<E> descendingIterator() {
      return delegate().descendingIterator(); // manually synchronized
    }

    @CheckForNull transient NavigableSet<E> descendingSet;

    @Override
    public NavigableSet<E> descendingSet() {

    @CheckForNull
    public E ceiling(E e) {
      synchronized (mutex) {
        return delegate().ceiling(e);
      }
    }

    @Override
    public Iterator<E> descendingIterator() {
      return delegate().descendingIterator(); // manually synchronized
    }

    @CheckForNull transient NavigableSet<E> descendingSet;

    @Override
    public NavigableSet<E> descendingSet() {

        final Supplier<String> pathSupplier = () -> {
            final boolean parallel = false;
            return StreamSupport.stream(((Iterable<ActivationFrame>) stk::descendingIterator).spliterator(), parallel)
                    .map(ActivationFrame::location)
                    .collect(Collectors.joining("."));
        };
        final Supplier<InputLocation> locationSupplier = () -> {