(VAR M N X )
(RULES 
        and(tt, X) -> activate(X)
        plus(N, 0) -> N
        plus(N, s(M)) -> s(plus(N, M))
        x(N, 0) -> 0
        x(N, s(M)) -> plus(x(N, M), N)
        activate(X) -> X
        
)