(VAR M N X X1 X2 )
(RULES 
        active(and(tt, X)) -> mark(X)
        active(plus(N, 0)) -> mark(N)
        active(plus(N, s(M))) -> mark(s(plus(N, M)))
        mark(and(X1, X2)) -> active(and(mark(X1), X2))
        mark(tt) -> active(tt)
        mark(plus(X1, X2)) -> active(plus(mark(X1), mark(X2)))
        mark(0) -> active(0)
        mark(s(X)) -> active(s(mark(X)))
        and(mark(X1), X2) -> and(X1, X2)
        and(X1, mark(X2)) -> and(X1, X2)
        and(active(X1), X2) -> and(X1, X2)
        and(X1, active(X2)) -> and(X1, X2)
        plus(mark(X1), X2) -> plus(X1, X2)
        plus(X1, mark(X2)) -> plus(X1, X2)
        plus(active(X1), X2) -> plus(X1, X2)
        plus(X1, active(X2)) -> plus(X1, X2)
        s(mark(X)) -> s(X)
        s(active(X)) -> s(X)
        
)