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

Lemma ex_filterdiff_scal : forall {F} {FF : ProperFilter F} (x : K * V),
is_filter_lim F x ->
(forall (n m : K), mult n m = mult m n) ->
ex_filterdiff (fun t : K * V => scal (fst t) (snd t)) F.