}

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

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

  @Override
  @CheckForNull
  public E floor(E element) {
    return forward.ceiling(element);
  }

  @Override
  @CheckForNull

  }

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

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

  @Override
  @CheckForNull
  public E floor(E element) {
    return forward.ceiling(element);
  }

  @Override
  @CheckForNull