Lemma filterdiff_RInt (f : R -> V) (If : R -> R -> V) (a b : R) : locally (a,b) (fun u : R * R => is_RInt f (fst u) (snd u) (If (fst u) (snd u))) -> continuous f a -> continuous f b -> filterdiff (fun u : R * R => If (fst u) (snd u)) (locally (a,b)) (fun u : R * R => minus (scal (snd u) (f b)) (scal (fst u) (f a))).