code: public Query setOptionalWords(List<String> words) {
      StringBuilder builder = new StringBuilder();
      for (String word : words) {
        builder.append(word);
        builder.append(",");
      }
      this.optionalWords = builder.toString();
      return this;
    }
docs: Set the list of words that should be considered as optional when found in
  the query.
  
  @param words The list of optional words.