static <A> AutoCloseable tempAddAll(Cl<A> l, Iterable<A> a) {
  if (l == null || a == null) null;
  new L<A> toRemove;
  for (A b : a)
    if (l.add(b))
      toRemove.add(b);
  ret empty(toRemove) ? null : -> l.removeAll(toRemove);
}

Began life as a copy of #1017722