// returns true if any changes
static <A> bool removeAllBut(Set<A> a, Collection<A> b) {
  bool changes;
  b = asSet(b);
  if (a != null) for (Iterator<A> it = a.iterator(); it.hasNext(); )
    if (!contains(b, it.next())) {
      it.remove();
      set changes;
    }
  ret changes;
}

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