[GiNaC-list] Multiplication of Boolean variables

Hi, I am implementing an arithmetic system for only Boolean variables based on GiNaC. May I know how I can modify the behaviour of symbolic multiplication such that: x*x == x (instead of x*x==x^2), and the multiplication of numeric remains unchanged (eg 3*4 == 12) ? Using subs(x^2 == x) works but I found it is slow if my expressions are huge and there are many symbols. Thank you very much. Best regards, argama