(VAR I P X Y Z )
(RULES 
        __(__(X, Y), Z) -> __(X, __(Y, Z))
        __(X, nil) -> X
        __(nil, X) -> X
        U11(tt) -> U12(tt)
        U12(tt) -> tt
        isNePal(__(I, __(P, I))) -> U11(tt)
        
)