* are off).
       */
      KeyList<K, V> keyList = requireNonNull(keyToKeyList.remove(node.getKey()));
      keyList.count = 0;
      modCount++;
    } else {
      // requireNonNull is safe (under the conditions listed in the comment in the branch above).
      KeyList<K, V> keyList = requireNonNull(keyToKeyList.get(node.getKey()));
      keyList.count--;

      if (node.previousSibling == null) {

     * Registers this extractor with the ExtractorFactory.
     * @param keyList The list of keys to register this extractor under.
     */
    public void register(final List<String> keyList) {
        if (keyList == null || keyList.isEmpty()) {
            throw new IllegalArgumentException("keyList must not be null or empty.");
        }
        getExtractorFactory().addExtractor(keyList, this);
    }

    /**

     *
     * @param keyList the list of keys to associate with the extractor
     * @param extractor the extractor to add
     * @throws CrawlerSystemException if the key list is null or empty, or if the extractor is null
     */
    public void addExtractor(final List<String> keyList, final Extractor extractor) {
        if (keyList == null || keyList.isEmpty()) {