static <A> L<A> cleanUpAndGetWeakReferencesList(L<WeakReference<A>> l) {
  if (l == null) null;
  synchronized(l) {
    new L<A> out;
    for (int i = 0; i < l(l); i++) {
      A a = l.get(i).get();
      if (a == null)
        l.remove(i--);
      else
        out.add(a);
    }
    ret out;
  }
}