Definition cond_positivity (x:R) : bool :=
match Rle_dec 0 x with
| left _ => true
| right _ => false
end.

(* A general purpose corollary. *)
Lemma cv_pow_half : forall a, Un_cv (fun n => a/2^n) 0.
intros a; unfold Rdiv; replace 0 with (a * 0) by ring.