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 behead_rcons {T : Type} (s : seq T) (t : T) : (0 < size s)%nat -> behead (rcons s t) = rcons (behead s) t.