static void printMultiSet(MultiSet ms) {
  printMultiSet(ms, null);
}

// renderer = func(O) -> S
static void printMultiSet(MultiSet ms, O renderer) {
  for (O o : ms.highestFirst())
    print(ms.get(o) + ": " + (renderer != null ? callF(renderer, o) : str(o)));
  print();
}