  void enable_packrat_parsing() {
    if (grammar_ != nullptr) {
      auto &rule = (*grammar_)[start_];
      rule.enablePackratParsing = true;
    }
  }
  void enable_packrat_parsing() {
    if (grammar_ != nullptr) {
      auto &rule = (*grammar_)[start_];
      rule.enablePackratParsing = true;
    }
  }