static LinkedHashSet<S> makeHull_optimized(MultiMap<S, S> map, S e) {
  new LinkedHashSet<S> l;
  makeHull_optimized_impl(map, e, l);
  ret l;
}

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