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

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

    transient @Nullable 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(f -> f.location)
                    .collect(Collectors.joining("."));
        };
        final Supplier<InputLocation> locationSupplier = () -> {

Lokio/internal/_BufferKt;
Lokio/internal/_ByteStringKt;
Lokio/internal/_PathKt;
PLandroidx/arch/core/internal/SafeIterableMap$DescendingIterator;-><init>(Landroidx/arch/core/internal/SafeIterableMap$Entry;Landroidx/arch/core/internal/SafeIterableMap$Entry;)V
PLandroidx/arch/core/internal/SafeIterableMap$DescendingIterator;->forward(Landroidx/arch/core/internal/SafeIterableMap$Entry;)Landroidx/arch/core/internal/SafeIterableMap$Entry;

  }

  @CollectionSize.Require(SEVERAL)
  public void testDescendingNavigation() {
    List<E> descending = new ArrayList<>();
    for (Iterator<E> i = navigableSet.descendingIterator(); i.hasNext(); ) {
      descending.add(i.next());
    }
    Collections.reverse(descending);
    assertEquals(values, descending);
  }

  public void testEmptySubSet() {

  }

  @CollectionSize.Require(SEVERAL)
  public void testDescendingNavigation() {
    List<E> descending = new ArrayList<>();
    for (Iterator<E> i = navigableSet.descendingIterator(); i.hasNext(); ) {
      descending.add(i.next());
    }
    Collections.reverse(descending);
    assertEquals(values, descending);
  }

  public void testEmptySubSet() {

    }

    @Override
    public NavigableSet<K> descendingSet() {
      return map().descendingKeySet();
    }

    @Override
    public Iterator<K> descendingIterator() {
      return descendingSet().iterator();
    }

    @Override
    public NavigableSet<K> subSet(
        @ParametricNullness K fromElement,
        boolean fromInclusive,

    }

    @Override
    public NavigableSet<K> descendingSet() {
      return map().descendingKeySet();
    }

    @Override
    public Iterator<K> descendingIterator() {
      return descendingSet().iterator();
    }

    @Override
    public NavigableSet<K> subSet(
        @ParametricNullness K fromElement,
        boolean fromInclusive,