static <A> Collection<A> combinedCollection(final Collection<A> a, final Collection<A> b) {
  if (empty(a)) ret b;
  if (empty(b)) ret a;
  
  ret new AbstractCollection<A> {
    public int size() { ret a.size() + b.size(); } // >= real value
    public bool contains(O o) { ret a.contains(o) || b.contains(o); }
    public Iterator<A> iterator() { fail("not implemented"); }
  };
}