@Override
 protected <T> MutableSortedBag<T> newWith(Comparator<? super T> comparator, T... elements) {
   return TreeBag.newBagWith(comparator, elements).asUnmodifiable();
 }
 @Override
 protected <T> MutableSortedBag<T> newWith(T... elements) {
   return TreeBag.newBagWith(elements).asUnmodifiable();
 }