docs: public void add(final ITopicNode specTopic, final String key) {
          if (specTopic == null) return;
  
          final Integer topicId = specTopic.getDBId();
          if (!topics.containsKey(topicId)) {
              topics.put(topicId, new LinkedList<ITopicNode>());
          }
  
          // Make sure the key exists
          if (!topicsKeys.containsKey(key)) {
              topicsKeys.put(key, new LinkedList<ITopicNode>());
          }
  
          topics.get(topicId).add(specTopic);
          topicsKeys.get(key).add(specTopic);
      }