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