static <A> bool setAddAllQuick(Collection<A> a, Collection<A> b) {
  Set<A> index = a instanceof Set ? (Set) a : new HashSet(a);
  bool change = false;
  for (A x : unnull(b))
    if (!index.contains(x)) {
      a.add(x);
      if (index != a) index.add(x);
      change = true;
    }
  ret change;
}