static <A> MultiSet<A> multiSetSetIntersection(MultiSet<A> a, Set<A> b) {
  MultiSet<A> ms = similarEmptyMultiSet(a);
  Set<A> bSet = asSet(b);
  for (A x : keys(a))
    if (bSet.contains(x))
      ms.add(x, a.get(x));
  ret ms;
}