static <A> L<L<A>> unrollTree(OccTree<A> tree) {
  new L<L<A>> l;
  unrollTree_collect(tree, new ArrayList, l);
  ret l;
}

static <A> void unrollTree_collect(OccTree<A> tree, L<A> l, L<L<A>> out) {
  for (A a : keys(tree.followUp)) {
    l.add(a);
    unrollTree_collect(tree.followUp(a), l, out);
    removeLast(l);
  }
  
  if (empty(tree.followUp))
    out.add(cloneList(l));
}