(** Remark: It is reasonable to think that [eq_rect_eq] is strictly stronger than [eq_rec_eq] (which is [eq_rect_eq] restricted on [Set]):  [Definition Eq_rec_eq := forall (P:U -> Set) (p:U) (x:P p) (h:p = p), x = eq_rec p P x p h.]  Typically, [eq_rect_eq] allows proving UIP and Streicher's K what does not seem possible with [eq_rec_eq]. In particular, the proof of [UIP] requires to use [eq_rect_eq] on [fun y -> x=y] which is in [Type] but not in [Set]. *)
End Equivalences.
(** UIP_refl is downward closed (a short proof of the key lemma of Voevodsky's proof of inclusion of h-level n into h-level n+1; see hlevelntosn in https://github.com/vladimirias/Foundations.git). *)

Theorem UIP_shift : forall U, UIP_refl_ U -> forall x:U, UIP_refl_ (x = x).
Proof (fun U UIP_refl x =>
@UIP_shift_on U x (UIP_refl x)).