*       should be rare.
   *   <li>The waiters list should be very short.
   * </ul>
   */
  private void removeWaiter(Waiter node) {
    node.thread = null; // mark as 'deleted'
    restart:
    while (true) {
      Waiter pred = null;
      Waiter curr = waiters;
      if (curr == Waiter.TOMBSTONE) {
        return; // give up if someone is calling complete
      }