Definition filterdiff (f : U -> V) F (l : U -> V) :=
  is_linear l /\ forall x, is_filter_lim F x ->
  is_domin F (fun y : U => minus y x) (fun y => minus (minus (f y) (f x)) (l (minus y x))).Definition ex_filterdiff (f : U -> V) F :=
  exists (l : U -> V), filterdiff f F l.

Lemma filterdiff_scal_r_fct : forall {F} {FF : Filter F} (k : K) (f lf : U -> V), (forall (n m : K), mult n m = mult m n) -> filterdiff f F lf -> filterdiff (fun x => scal k (f x)) F (fun x => scal k (lf x)).