private final Set<String> stopTags = new HashSet<>();

    public KuromojiPartOfSpeechFilterFactory(IndexSettings indexSettings, Environment env, String name, Settings settings) {
        super(indexSettings, name, settings);
        List<String> wordList = Analysis.parseWordList(env, settings, "stoptags", s -> s);
        if (wordList != null) {
            stopTags.addAll(wordList);
        } else {