static <A, B> MultiSetMap<B, A> reversedOrderedMultiSetMap(MultiMap<A, B> map) {
  MultiSetMap<B, A> mm = orderedMultiSetMap();
  for (A key : keys(map))
    for (B value : map.get(key))
      mm.put(value, key);
  ret mm;
}

Began life as a copy of #1034843