intros f x l a df;
apply (derivable_pt_lim_ext (fun y => a * f y)).
- intros z; rewrite Rmult_comm; reflexivity.
- unfold Rdiv; rewrite Rmult_comm; apply derivable_pt_lim_scal; assumption.
Qed.

(** ** Rules for derivable_pt (derivability at a point) *)
Lemma derivable_pt_id : forall x:R, derivable_pt id x.