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 foldr_rcons {T T0 : Type} : forall (f : T0 -> T -> T) x0 s t, foldr f x0 (rcons s t) = foldr f (f t x0) s.