static int iterativeSum(Tree<Integer> t) {
  if (t == null)
    return 0;

  int sum = 0;

  Stack<Tree<Integer>> s = new Stack<Tree<Integer>>();
  s.push(t);

  Tree<Integer> tr;
  Set<Tree<Integer>> subtrees;
  while (!s.isEmpty()) {
    tr = s.pop();
    sum += tr.elem();

    subtrees = tr.subtrees();
    for (Tree<Integer> subtree : subtrees) {
        s.push(subtree);
    }
  }

  return sum;
}