friend istream& operator >> (istream& is, zFrac &r) {
        char ch;
        is >> r.num;
        auto fl = is.flags();
        is.unsetf(ios_base::skipws);
        is >> ch;
        if (ch == '/' || ch == '\\')
            is >> r.denom;
        else {
            is.putback(ch);
            r.denom = 1;
        }
        zFrac f;
        is.setf(ios_base::skipws);
        r.norm();
        return is;
    }