[GiNaC-list] same expression prints in differs orders

Hi, I am try to test expressions with "match", but it not works always with the same expression. I think that the problem is at internal representation or at internal order. When I debug the code, same expression prints differ. ex test = pow(log(x),2)/x; cout << test; // sometimes x^(-1)*log(x)^2 // other sometimes log(x)^2*x^(-1) How I can force GiNaC to unique representation? thanks