Section recursive.
Variable A:Type.
Variable ag:ageable A.
Variable f:pred A -> pred A.
Variable Hc : contractive f.
End recursive.
Definition Rec {A} `{ageable A} f : pred A
:= HORec (fun x _ => f (x tt)) tt.

Lemma Rec_fold_unfold : forall {A} `{ageable A} f (H:contractive f),
Rec f = f (Rec f).