void CustomElementReactionStack::enqueue(Member<ElementQueue>& queue,
                                         Element* element,
                                         CustomElementReaction* reaction) {
  if (!queue)
    queue = new ElementQueue();
  queue->append(element);

  CustomElementReactionQueue* reactions = m_map.get(element);
  if (!reactions) {
    reactions = new CustomElementReactionQueue();
    m_map.add(element, reactions);
  }

  reactions->add(reaction);
}