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(* 02110-1301 USA                                                     *)
(* Contribution to the Coq Library   V6.3 (July 1999)                    *)
(* This is a step towards inaccessible cardinals                *)
(* We can define "small" sets, using the Type's hierarchy       *)
(* Since Coq does not support universe polymorphism, we need    *)
(* to duplicate some definitions here.                          *)
(* The small sets  *)
Inductive Ens' : Type :=
sup' : forall A : Type, (A -> Ens') -> Ens'.
(* Existential quantification *)
Inductive EXType' (P : Type) (Q : P -> Prop) : Prop :=
EXTypei' : forall x : P, Q x -> EXType' P Q.
(* Cartesian product in Type *)
Inductive prod_t' (A B : Type) : Type :=
pair_t' : A -> B -> prod_t' A B.
(* Existential on the Type level *)
Inductive depprod'' (A : Type) (P : A -> Type) : Type :=
dep_i'' : forall x : A, P x -> depprod'' A P.
(* Recursive Definition of the extentional equality on sets *)
Definition EQ' : Ens' -> Ens' -> Prop.
simple induction 1; intros A f eq1.
simple induction 1; intros B g eq2.
apply and.
exact (forall x : A, EXType' _ (fun y : B => eq1 x (g y))).
exact (forall y : B, EXType' _ (fun x : A => eq1 x (g y))).
Defined.
(* small sets can be injected into big sets             *)
Definition inj : Ens' -> Ens.
simple induction 1; intros A f fr.
exact (sup A fr).
Defined.

Theorem inj_sound : forall E1 E2 : Ens', EQ' E1 E2 -> EQ (inj E1) (inj E2).