View Compilation Error

0_0_33906201_7283.cpp: In function 'int main()':
0_0_33906201_7283.cpp:96:30: error: invalid initialization of non-const reference of type 'mat& {aka std::vector<std::vector<int> >&}' from an rvalue of type 'mat {aka std::vector<std::vector<int> >}'
    B = mul(power(A, n - 2), B);
                              ^
0_0_33906201_7283.cpp:45:5: note: in passing argument 1 of 'mat mul(mat&, mat&)'
 mat mul(mat &A, mat &B) {
     ^