// Textbook fragment 07.13
  public static <E> int diskSpace (Tree<E> T, Position<E> v) {
    int s = size(v);            // start with the size of the node itself
    for (Position<E> w :  T.children(v))
      // add the recursively computed space used by the children of v
      s += diskSpace(T, w);
    if (T.isInternal(v)) {
      // print name and disk space used
      System.out.print(name(v) + ": " + s);
      }
    return s;
  }