(VAR f x xs ys )
(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(flatten, app(app(node, x), xs)) -> app(app(cons, x), app(concat, app(app(map, flatten), xs)))
        app(concat, nil) -> nil
        app(concat, app(app(cons, x), xs)) -> app(app(append, x), app(concat, xs))
        app(app(append, nil), xs) -> xs
        app(app(append, app(app(cons, x), xs)), ys) -> app(app(cons, x), app(app(append, xs), ys))
        
)