static <A, B extends A> Set<A> setMinusSet(Set<A> l, Collection<B> stuff) {
  if (empty(stuff)) ret l;
  Set<B> set = asSet(stuff);
  Set<A> l2 = similarEmptySet(l);
  for (A a : l) if (!set.contains(a)) l2.add(a);
  ret l2;
}

static <A, B extends A> Set<A> setMinusSet(Cl<A> l, Collection<B> stuff) {
  ret setMinusSet(asSet(l), stuff);
}