View Compilation Error

0_0_30085702_22933.cpp:53:5: error: prototype for 'mat mat::operator*(mat&)' does not match any in class 'mat'
 mat mat::operator *(mat& a)
     ^
0_0_30085702_22933.cpp:51:7: error: candidate is: mat& mat::operator*(mat&)
  mat& operator *(mat&);
       ^