static <A, B> MultiMap<B, A> reverseMapAsMultiMap(Map<A, B> map) {
  ret reverseMapAsMultiMap(map, new MultiMap);
}

static <A, B> MultiMap<B, A> reverseMapAsMultiMap(Map<A, B> map, MultiMap<B, A> mm) {
  for (A key : keys(map))
    mm.put(map.get(key), key);
  ret mm;
}