static <A, B> L<B> mapMultiSet(MultiSet<A> ms, F2<A, Int, B> f) {
  new L<B> l;
  if (ms != null) for (A key : ms.keySet())
    l.add(callF(f, key, ms.get(key)));
  ret l;
}

static <A, B> L<B> mapMultiSet(MultiSet<A> ms, IF2<A, Int, B> f) {
  new L<B> l;
  if (ms != null) for (A key : ms.keySet())
    l.add(callF(f, key, ms.get(key)));
  ret l;
}