View Compilation Error

0_0_21719384_5058.cpp: In function 'int main()':
0_0_21719384_5058.cpp:122:12: error: no match for 'operator*' (operand types are 'Mat' and 'Mat')
   Mat c = a*pow(b*a, n*n - 1)*b;
            ^
0_0_21719384_5058.cpp:122:12: note: candidate is:
0_0_21719384_5058.cpp:87:6: note: Mat Mat::operator*(Mat&)
  Mat operator*(Mat &a)
      ^
0_0_21719384_5058.cpp:87:6: note:   no known conversion for argument 1 from 'Mat' to 'Mat&'