namespace detail {
  struct any { template<typename T> any(T &&) {} };
  template<typename T, typename U> struct fst { typedef T type; };
  template<typename ...Ts> struct select_impl {
    template<typename U, typename ...Vs>
    static U &&select(typename fst<any, Ts>::type..., U &&u, Vs &&...) {
      return static_cast<U&&>(u);
    }
  };
}

template<typename T, typename ...Ts>
auto back(T &&t, Ts &&...ts) -> decltype(detail::select_impl<Ts...>::select(static_cast<T&&>(t), static_cast<Ts&&>(ts)...)) {
  return detail::select_impl<Ts...>::select(static_cast<T&&>(t), static_cast<Ts&&>(ts)...);
}