static <A, B extends A> Set<A> setDiff_bothWays(Cl<A> a, Cl<B> b) {
  a = asSet(a);
  b = asSet(b);
  Set<A> out = similarEmptySet(a);
  fOr (A x : a)
    if (!contains(b, x))
      out.add(x);
  fOr (A x : b)
    if (!contains(a, x))
      out.add(x);
  ret out;
}