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.

Lemma derivable_pt_xeq:
forall (f : R -> R) (x1 x2 : R), x1=x2 -> derivable_pt f x1 -> derivable_pt f x2.