@GwtIncompatible("NavigableSet")
  public UnmodifiableIterator<E> descendingIterator() {
    return forward.iterator();
  }

  @Override
  @GwtIncompatible("NavigableSet")
  ImmutableSortedSet<E> createDescendingSet() {
    throw new AssertionError("should never be called");
  }

  @Override
  public @Nullable E lower(E element) {
    return forward.higher(element);
  }

  @Override

    throw new InvalidObjectException("Use SerializedForm");
  }

  @GwtIncompatible // NavigableSet
  @Override
  ImmutableSortedSet<C> createDescendingSet() {
    return ImmutableSortedSet.emptySet(Ordering.<C>natural().reverse());
  }