static <A> Set<A> setIntersection(Cl<A> a, Cl<A> b) {
  Set<A> set = similarEmptySet(a);
  if (nempty(a) && nempty(b)) {
    Set<A> bSet = asSet(b);
    for (A x : a) if (bSet.contains(x)) set.add(x);
  }
  ret set;
}