T det() const {
        assert(size().first == size().second);
        std::vector<int> perm(size().first);
        for (int i = 0; i < perm.size(); ++i) perm[i] = i;
        T ans = 0;
        while (true) {
            T add = 1;
            for (int i = 0; i < perm.size(); ++i)
                add *= this->operator()(i, perm[i]);
            for (int i = 0; i < perm.size(); ++i)
                for (int j = i + 1; j < perm.size(); ++j)
                    if (perm[i] > perm[j]) add = -add;
            //for (auto i : perm) std :: cout << i << ' ';
            //std :: cout << add << '\n';
            ans += add;
            if (!next_permutation(perm.begin(), perm.end())) break;
        }
        return ans;
    }