Definition pos_div_2 (eps : posreal) := mkposreal _ (is_pos_div_2 eps).(** * The sign function *)

Definition sign (x : R) :=
  match total_order_T 0 x with
  | inleft (left _) => 1
  | inleft (right _) => 0
  | inright _ => -1
  end.

Lemma unzip2_rcons {S T : Type} (s : seq (S*T)) (h : S*T) : unzip2 (rcons s h) = rcons (unzip2 s) (snd h).