static Lisp snlQuestionMarksToBack(Lisp tree) {
  if (tree == null || tree.size() == 0) ret tree;

  if (tree.isA("<") && tree.get(0).is("?", "."))
    ret lisp("* " + tree.get(0).head,
      snlQuestionMarksToBack(tree.get(1)));

  // recurse
  Lisp lisp = new Lisp(tree.head);
  for (Lisp child : tree)
    lisp.add(snlQuestionMarksToBack(child));
  ret lisp;
}