static <A> A msOneOf(MultiSet<A> ms) {
  int total = ms.size();
  if (total == 0) ret null;
  int n = random(total);

  for (Map.Entry<A, Integer> entry : ms.map.entrySet()) {
    n -= entry.getValue();
    if (n < 0)
      return entry.getKey();
  }
  
  ret null;
}