(VAR I P X X1 X2 Y Z )
(RULES 
        active(__(__(X, Y), Z)) -> mark(__(X, __(Y, Z)))
        active(__(X, nil)) -> mark(X)
        active(__(nil, X)) -> mark(X)
        active(U11(tt)) -> mark(U12(tt))
        active(U12(tt)) -> mark(tt)
        active(isNePal(__(I, __(P, I)))) -> mark(U11(tt))
        active(__(X1, X2)) -> __(active(X1), X2)
        active(__(X1, X2)) -> __(X1, active(X2))
        active(U11(X)) -> U11(active(X))
        active(U12(X)) -> U12(active(X))
        active(isNePal(X)) -> isNePal(active(X))
        __(mark(X1), X2) -> mark(__(X1, X2))
        __(X1, mark(X2)) -> mark(__(X1, X2))
        U11(mark(X)) -> mark(U11(X))
        U12(mark(X)) -> mark(U12(X))
        isNePal(mark(X)) -> mark(isNePal(X))
        proper(__(X1, X2)) -> __(proper(X1), proper(X2))
        proper(nil) -> ok(nil)
        proper(U11(X)) -> U11(proper(X))
        proper(tt) -> ok(tt)
        proper(U12(X)) -> U12(proper(X))
        proper(isNePal(X)) -> isNePal(proper(X))
        __(ok(X1), ok(X2)) -> ok(__(X1, X2))
        U11(ok(X)) -> ok(U11(X))
        U12(ok(X)) -> ok(U12(X))
        isNePal(ok(X)) -> ok(isNePal(X))
        top(mark(X)) -> top(proper(X))
        top(ok(X)) -> top(active(X))
        
)