docs: private QNode getValidatedTail() {
      for (;;) {
        QNode h = this.head.get();
        QNode first = h.next;
        if (first != null && first.next == first) { // help advance
          advanceHead(h, first);
          continue;
        }
        QNode t = this.tail.get();
        QNode last = t.next;
        if (t == this.tail.get()) {
          if (last != null) {
            this.tail.compareAndSet(t, last); // help advance
          } else {
            return t;
          }
        }
      }
    }