// synchronized block, so we can iterate the list without the lock held here.
    RunnableExecutorPair reversedList = null;
    while (list != null) {
      RunnableExecutorPair tmp = list;
      list = list.next;
      tmp.next = reversedList;
      reversedList = tmp;
    }
    while (reversedList != null) {
      executeListener(reversedList.runnable, reversedList.executor);
      reversedList = reversedList.next;
    }
  }

    Listener head = ATOMIC_HELPER.gasListeners(this, Listener.TOMBSTONE);
    Listener reversedList = onto;
    while (head != null) {
      Listener tmp = head;
      head = head.next;
      tmp.next = reversedList;
      reversedList = tmp;
    }
    return reversedList;
  }

  // TODO(user): move parts into a default method on ListenableFuture?
  @Override
  public String toString() {