void subscribeNext() {
      Observable<? extends T> t;
      synchronized (guard) {
        t = queue.peek();
        if (t == null || active >= maxConcurrency) {
          return;
        }
        active++;
        queue.poll();
      }

      MergeItemSubscriber itemSub = new MergeItemSubscriber(SOURCE_INDEX.getAndIncrement(this));
      subscribers.add(itemSub);

      csub.add(itemSub);

      WIP.incrementAndGet(this);

      t.unsafeSubscribe(itemSub);

      request(1);
    }
 int incRefCnt() {
   return refcntUpdater.incrementAndGet(this);
 }