View Compilation Error

0_0_30085733_11836.cpp:53:6: error: prototype for 'mat& mat::operator*(mat&)' does not match any in class 'mat'
 mat &mat::operator *(mat& a)
      ^
0_0_30085733_11836.cpp:51:6: error: candidate is: mat mat::operator*(mat&)
  mat operator *( mat&);
      ^