Listener oldHead = listeners();
      if (oldHead != Listener.TOMBSTONE) {
        Listener newNode = new Listener(listener, executor);
        do {
          newNode.next = oldHead;
          if (casListeners(oldHead, newNode)) {
            return;
          }
          oldHead = listeners(); // re-read
        } while (oldHead != Listener.TOMBSTONE);
      }
    }