(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))
        
)