(VAR f x xs xss ys yss )
(RULES 
        app(app(map, f), nil) -> nil
        app(app(map, f), app(app(cons, x), xs)) -> app(app(cons, app(f, x)), app(app(map, f), xs))
        app(app(append, xs), nil) -> xs
        app(app(append, nil), ys) -> ys
        app(app(append, app(app(cons, x), xs)), ys) -> app(app(cons, x), app(app(append, xs), ys))
        app(app(zip, nil), yss) -> yss
        app(app(zip, xss), nil) -> xss
        app(app(zip, app(app(cons, xs), xss)), app(app(cons, ys), yss)) -> app(app(cons, app(app(append, xs), ys)), app(app(zip, xss), yss))
        app(app(combine, xs), nil) -> xs
        app(app(combine, xs), app(app(cons, ys), yss)) -> app(app(combine, app(app(zip, xs), ys)), yss)
        app(levels, app(app(node, x), xs)) -> app(app(cons, app(app(cons, x), nil)), app(app(combine, nil), app(app(map, levels), xs)))
        
)