* nodes. The traversal order among descendant nodes (particularly aunts and nieces) are
     * determined by the {@code InsertionOrder} parameter: nieces are placed at the FRONT before
     * aunts for pre-order; while in BFS they are placed at the BACK after aunts.
     */
    private Iterator<N> topDown(Iterator<? extends N> startNodes, InsertionOrder order) {
      Deque<Iterator<? extends N>> horizon = new ArrayDeque<>();