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

  // operation in the constructor.
  //
  // 1. In the listener that performs the callback. In this case it is fine since inputFuture is
  //    assigned prior to calling addListener, and addListener happens-before any invocation of the
  //    listener. Notably, this means that 'volatile' is unnecessary to make 'inputFuture' visible
  //    to the listener.
  //

      return ticker;
    }
    return recordsTime ? Ticker.systemTicker() : NULL_TICKER;
  }

  /**
   * Specifies a listener instance that caches should notify each time an entry is removed for any
   * {@linkplain RemovalCause reason}. Each cache created by this builder will invoke this listener
   * as part of the routine maintenance described in the class documentation above.
   *