#include <string>
    #include <iostream> // for the demo only

    std::string concat(std::string const& a) {
      return a;
    }

    template <typename... Items>
    std::string concat(std::string const& a, std::string const& b, Items&&... args) {
      return concat(a + b, args...);
    }

    int main() {
      std::cout << concat("0", "1", "2", "3") << "\n";
    }