static <A> void removeAllBut(Set<A> a, Collection<A> b) {
  if (a != null) for (Iterator<A> it = a.iterator(); it.hasNext(); )
    if (!contains(b, it.next()))
      it.remove();
}

static <A, B> void removeAllBut(Map<A, B> a, Collection<A> b) {
  if (a != null) removeAllBut(a.keySet(), b);
}