static L<S> makeHull(MultiMap<S, S> map, S e) {
  new L<S> l;
  makeHull_impl(map, e, l);
  ret l;
}

static void makeHull_impl(MultiMap<S, S> map, S e, L<S> dest) {
  if (dest.contains(e))
    ret;
  dest.add(e);
  for (S f : map.get(e))
    makeHull_impl(map, f, dest);
}