static Set<S> intersectCISets(Cl<S> a, Cl<S> b) {
  ret setIntersection(asCISet(a), asCISet(b));
}

static <A extends Cl<S>> Set<S> intersectCISets(Cl<A> l) {
  Set<S> set = null;
  fOr (Cl<S> x : l)
    set = set == null ? asCISet(x) : intersectCISets(set, x);
  ret set;
}