// Textbook fragment 07.07
  public static <E> int height2 (Tree<E> T, Position<E> v) {
    if (T.isExternal(v)) return 0;
    int h = 0;
    for (Position<E> w : T.children(v))
      h = Math.max(h, height2(T, w));
    return 1 + h;
  }