// recursively apply tokenizer
    MessageSegment _applyTokenizerToText(
        Function tokenizerFunction(String str), MessageSegment base) {
      List<MessageSegment> newList = new List<MessageSegment>();
      if (base.subSegemnts != null) {
        for (MessageSegment child in base.subSegemnts) {
          if (child.type == 'text') {
            newList.addAll(tokenizerFunction(child.data));
          } else {
            _applyTokenizerToText(tokenizerFunction, child);
            newList.add(child);
          }
        }
      }
      base.subSegemnts = newList;
    }