static S snlApply(S snl, Map<S, Lisp> matches) {
  ret snlFromTree(snlApply(snlToTree(snl), matches));
}

static Lisp snlApply(Lisp tree, Map<S, Lisp> matches) {
  if (tree == null) ret null;
  if (tree.isEmpty())
    ret or(matches.get(tree.head), tree);

  Lisp lisp = new Lisp(tree.head);
  for (Lisp child : tree)
    lisp.add(snlApply(child, matches));
  ret lisp;
}