public void Subscribe(ISubscriber <T> s)
        {
            var parent = new SortedJoinSubscription(s, source.Parallelism, comparer);

            s.OnSubscribe(parent);
            source.Subscribe(parent.subscribers);
        }
 internal InnerSubscriber(SortedJoinSubscription parent, int index)
 {
     this.parent = parent;
     this.index  = index;
 }