template<typename T_container, typename T = typename enable_if<!is_same<T_container, string>::value, typename T_container::value_type>::type>

ostream& operator<<(ostream &os, const T_container &v) {

    os << '{';

    string sep;

    for (const T &x : v)

        os << sep << x, sep = ", ";

    return os << '}';

}