(VAR N X X1 X2 XS )
(RULES 
        active(from(X)) -> mark(cons(X, from(s(X))))
        active(after(0, XS)) -> mark(XS)
        active(after(s(N), cons(X, XS))) -> mark(after(N, XS))
        mark(from(X)) -> active(from(mark(X)))
        mark(cons(X1, X2)) -> active(cons(mark(X1), X2))
        mark(s(X)) -> active(s(mark(X)))
        mark(after(X1, X2)) -> active(after(mark(X1), mark(X2)))
        mark(0) -> active(0)
        from(mark(X)) -> from(X)
        from(active(X)) -> from(X)
        cons(mark(X1), X2) -> cons(X1, X2)
        cons(X1, mark(X2)) -> cons(X1, X2)
        cons(active(X1), X2) -> cons(X1, X2)
        cons(X1, active(X2)) -> cons(X1, X2)
        s(mark(X)) -> s(X)
        s(active(X)) -> s(X)
        after(mark(X1), X2) -> after(X1, X2)
        after(X1, mark(X2)) -> after(X1, X2)
        after(active(X1), X2) -> after(X1, X2)
        after(X1, active(X2)) -> after(X1, X2)
        
)