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