View Compilation Error

0_0_17022543_6794.cpp: In function 'Matrix Pow(int)':
0_0_17022543_6794.cpp:36:32: error: invalid initialization of non-const reference of type 'Matrix&' from an rvalue of type 'Matrix'
         return Mul(mat,Pow(n-1));
                                ^
0_0_17022543_6794.cpp:18:8: note: in passing argument 2 of 'Matrix Mul(Matrix&, Matrix&)'
 Matrix Mul(Matrix &a,Matrix &b){
        ^