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 foldr_noSeed intersectCISets((Cl<Set<S>>) (Cl) l);
}