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) {
  ret (Set) foldr_noSeed intersectCISets(l);
}