Local Ltac plus_grab e :=
repeat rewrite (plus_assoc _ e) -(plus_comm e) -(plus_assoc e).

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