foo.cc: In function 'std::vector<std::vector<long long unsigned int> > mul(std::vector<std::vector<long long unsigned int> >&, std::vector<std::vector<long long unsigned int> >&)':
foo.cc:20:13: warning: unused variable 'm_b' [-Wunused-variable]
   20 |         int m_b = b.size(), n_b = b[0].size();
      |             ^~~
foo.cc: In function 'std::vector<std::vector<long long unsigned int> > solve(ull, ull, ull, ull, ull)':
foo.cc:68:23: error: cannot bind non-const lvalue reference of type 'std::vector<std::vector<long long unsigned int> >&' to an rvalue of type 'std::vector<std::vector<long long unsigned int> >'
   68 |         return mul(pow(m, T), x);
      |                    ~~~^~~~~~
foo.cc:14:46: note:   initializing argument 1 of 'std::vector<std::vector<long long unsigned int> > mul(std::vector<std::vector<long long unsigned int> >&, std::vector<std::vector<long long unsigned int> >&)'
   14 | vector<vector<ull>> mul(vector<vector<ull>>& a, vector<vector<ull>>& b) {
      |                         ~~~~~~~~~~~~~~~~~~~~~^

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