(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))
        active(from(X)) -> from(active(X))
        active(cons(X1, X2)) -> cons(active(X1), X2)
        active(s(X)) -> s(active(X))
        active(after(X1, X2)) -> after(active(X1), X2)
        active(after(X1, X2)) -> after(X1, active(X2))
        from(mark(X)) -> mark(from(X))
        cons(mark(X1), X2) -> mark(cons(X1, X2))
        s(mark(X)) -> mark(s(X))
        after(mark(X1), X2) -> mark(after(X1, X2))
        after(X1, mark(X2)) -> mark(after(X1, X2))
        proper(from(X)) -> from(proper(X))
        proper(cons(X1, X2)) -> cons(proper(X1), proper(X2))
        proper(s(X)) -> s(proper(X))
        proper(after(X1, X2)) -> after(proper(X1), proper(X2))
        proper(0) -> ok(0)
        from(ok(X)) -> ok(from(X))
        cons(ok(X1), ok(X2)) -> ok(cons(X1, X2))
        s(ok(X)) -> ok(s(X))
        after(ok(X1), ok(X2)) -> ok(after(X1, X2))
        top(mark(X)) -> top(proper(X))
        top(ok(X)) -> top(active(X))
        
)