exact is_series_scal.
Qed.

(* Specifications: *)
Lemma ex_series_scal (c : K) (a : nat -> V) :
ex_series a -> ex_series (fun n => scal c (a n)).
Lemma ex_series_scal_l : forall (c : K) (a : nat -> V),
ex_series a -> ex_series (fun n => scal c (a n)).