let p_xor p1 p2 = match val_of p1, val_of p2 with | Ptrue , Ptrue -> cut (cut Pfalse p2) p1 | Ptrue ,_ -> cut Ptrue p1 | _,Ptrue -> cut Ptrue p2 | Pfalse , _ -> p2 | _ , Pfalse -> p1 | _ -> Pnot(Piff(p1,p2))