(VAR N X XS )
(RULES 
        from(X) -> cons(X, n__from(n__s(X)))
        after(0, XS) -> XS
        after(s(N), cons(X, XS)) -> after(N, activate(XS))
        from(X) -> n__from(X)
        s(X) -> n__s(X)
        activate(n__from(X)) -> from(activate(X))
        activate(n__s(X)) -> s(activate(X))
        activate(X) -> X
        
)