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(* 02110-1301 USA                                                     *)
(* Contribution to the Coq Library   V6.3 (July 1999)                    *)
(************************************************************************)
(*                      Zermolo Set Theory                              *)
(*                                                                      *)
(*                        Benjamin Werner                               *)
(************************************************************************)
(* This is an encoding of usual Set Theory, simillar to Peter Aczel's work *)
(* in the early 80's. The main difference is that the propositions here    *)
(* live in the impredicative world of "Prop". Thus, priority is given to   *)
(* expressivity against constructivity.                                    *)
(* Since the definition of Sets is the same for both approaches, I added   *)
(* most  of Aczel's encoding of CZF at the end of the file. Many           *)
(* definitions are common to both aproaches.                               *)
Set Asymmetric Patterns.
(* The type representing sets  (Ensemble = french for set) *)
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.
(* Membership on sets *)
Definition IN (E1 E2 : Ens) : Prop :=
match E2 with
| sup A f => EXType _ (fun y : A => EQ E1 (f y))
end.
(* INCLUSION *)
Definition INC : Ens -> Ens -> Prop.
intros E1 E2.
exact (forall E : Ens, IN E E1 -> IN E E2).
Defined.
(* EQ is an equivalence relation  *)
simple induction E.
intros A f HR.
simpl in |- *.
split; intros.
exists x; auto.
exists y; auto.
Qed.
simple induction E1; intros A1 f1 r1; simple induction E2; intros A2 f2 r2;
simple induction E3; intros A3 f3 r3; simpl in |- *;
intros e1 e2.
split; (elim e1; intros I1 I2; elim e2; intros I3 I4).
intros a1; elim (I1 a1); intros a2.
elim (I3 a2); intros a3.
exists a3.
apply r1 with (f2 a2); auto.
intros a3; elim (I4 a3); intros a2; elim (I2 a2); intros a1; exists a1.
apply r1 with (f2 a2); auto.
Qed.
simple induction E1; intros A1 f1 r1; simple induction E2; intros A2 f2 r2;
simpl in |- *; simple induction 1; intros e1 e2; split.
intros a2; elim (e2 a2); intros a1 H1; exists a1; auto.
intros a1; elim (e1 a1); intros a2 H2; exists a2; auto.
Qed.
simple induction E; intros A f r; simple induction E'; intros A' f' r';
simpl in |- *; simple induction 1; intros e1 e2; unfold INC in |- *;
simpl in |- *.
intros C; simple induction 1; intros a ea; elim (e1 a); intros a' ea';
exists a'.
apply EQ_tran with (f a); assumption.
Qed.
Hint Resolve EQ_sym EQ_refl EQ_INC: zfc.
(* easy lemma *)
simple induction E; intros A f r; simple induction E'; intros A' f' r';
unfold INC in |- *; simpl in |- *; intros I1 I2; split.
intros a; apply I1.
exists a; auto with zfc.
intros a'; cut (EXType A (fun x : A => EQ (f' a') (f x))).
simple induction 1; intros a ea; exists a; auto with zfc.
apply I2; exists a'; auto with zfc.
Qed.
Hint Resolve INC_EQ: zfc.
(* Membership is extentional (i.e. is stable w.r.t. EQ)   *)
simple induction E''; intros A'' f'' r'' e; simpl in |- *; simple induction 1;
intros a'' p; exists a''; apply EQ_tran with E; auto with zfc.
Qed.
simple induction E'; intros A' f' r'; simple induction E'';
intros A'' f'' r''; simpl in |- *; simple induction 1;
intros e1 e2; simple induction 1; intros a' e'; elim (e1 a');
intros a'' e''; exists a''; apply EQ_tran with (f' a');
assumption.
Qed.
(* Inclusion is reflexive, transitive, extentional *)
unfold INC in |- *; auto with zfc.
Qed.
unfold INC in |- *; auto with zfc.
Qed.
simple induction E''; unfold INC in |- *; simpl in |- *;
intros A f HR e H1 E0 i; apply H1.
apply IN_sound_right with E'; auto with zfc.
Qed.
simple induction E'; simple induction E''; unfold INC in |- *; simpl in |- *;
intros.
elim (H2 E0); try assumption; intros.
elim H1; intros HA HB; elim (HA x); intros.
exists x0; apply EQ_tran with (e x); auto with zfc.
Qed.
(* Useful types (actually top and bottom)   *)
Inductive Un : Set :=
void : Un.
Inductive F : Set :=.
(* The empty set  (vide = french for empty)   *)
Definition Vide : Ens := sup F (fun f : F => match f return Ens with
end).
(* The axioms of the empty set *)
unfold Vide in |- *; simpl in |- *; intros E H; cut False.
simple induction 1.
elim H; intros x; elim x.
Qed.
unfold Vide in |- *; simple induction E; simpl in |- *; intros A e H H0;
split.
intros; elim (H0 (e x)); auto with zfc.
exists x; auto with zfc.
simple induction y.
Qed.
(* Pair *)
Definition Paire : forall E E' : Ens, Ens.
intros.
apply (sup bool).
simple induction 1.
exact E.
exact E'.
Defined.
(* The pair construction is extentional *)
unfold Paire in |- *.
simpl in |- *.
intros; split.
simple induction x.
exists true; auto with zfc.
exists false; auto with zfc.
simple induction y; simpl in |- *.
exists true; auto with zfc.
exists false; auto with zfc.
Qed.
unfold Paire in |- *; simpl in |- *; intros; split.
simple induction x.
exists true; auto with zfc.
exists false; auto with zfc.
simple induction y.
exists true; auto with zfc.
exists false; auto with zfc.
Qed.
Hint Resolve Paire_sound_right Paire_sound_left: zfc.
(* The axioms of the pair *)
unfold Paire in |- *; simpl in |- *; exists true; simpl in |- *;
auto with zfc.
Qed.
unfold Paire in |- *; simpl in |- *; exists false; simpl in |- *;
auto with zfc.
Qed.
unfold Paire in |- *; simpl in |- *.
simple induction 1; intros b; elim b; simpl in |- *; auto with zfc.
Qed.
Hint Resolve IN_Paire_left IN_Paire_right Vide_est_vide: zfc.
(* The singleton set  *)
(* Note that we could define it directly using the base type Un *)
Definition Sing (E : Ens) := Paire E E.
(* The axioms  *)
unfold Sing in |- *; auto with zfc.
Qed.
unfold Sing in |- *; intros E E' H; elim (Paire_IN E' E' E);
auto with zfc.
Qed.
Hint Resolve IN_Sing IN_Sing_EQ: zfc.
unfold Sing in |- *; intros; apply EQ_tran with (Paire A A');
auto with zfc.
Qed.
Hint Resolve Sing_sound: zfc.
intros; cut (IN E1 (Sing E2)).
intros; auto with zfc.
apply IN_sound_right with (Sing E1); auto with zfc.
Qed.
Hint Resolve EQ_Sing_EQ: zfc.
(* We here need sigma types -- i.e. computational existentials *)
Inductive sig (A : Type) (P : A -> Prop) : Type :=
exist : forall x : A, P x -> sig A P.
(* The set obtained by the comprehension (or separation) axiom *)
Definition Comp : Ens -> (Ens -> Prop) -> Ens.
simple induction 1; intros A f fr P.
apply (sup (sig A (fun x => P (f x)))).
simple induction 1; intros x p; exact (f x).
Defined.
(* The comprehension/separation axioms *)
unfold Comp, INC in |- *; simple induction E; simpl in |- *; intros.
elim H0; simple induction x; intros; exists x0; auto with zfc.
Qed.
simple induction E; simpl in |- *; intros B f Hr A P H i; elim i; intros c;
elim c; simpl in |- *; intros x q e; apply H with (f x);
auto with zfc.
Qed.
simple induction E; simpl in |- *; intros B f HR A P H i; elim i;
simpl in |- *; intros.
cut (P (f x)).
intros Pf.
exists (exist B (fun x : B => P (f x)) x Pf); simpl in |- *;
auto with zfc.
apply H with A; auto with zfc.
Qed.
(* Again, extentionality is not stated, but easy *)
(* Projections of a set: *)
(*  1: its base type     *)
Definition pi1 : Ens -> Type.
simple induction 1.
intros A f r.
exact A.
Defined.
(*  2: the function      *)
Definition pi2 : forall E : Ens, pi1 E -> Ens.
simple induction E.
intros A f r.
exact f.
Defined.
(* The Union set   *)
Definition Union : forall E : Ens, Ens.
simple induction 1; intros A f r.
apply (sup (depprod A (fun x : A => pi1 (f x)))).
simple induction 1; intros a b.
exact (pi2 (f a) b).
Defined.

Theorem EQ_EXType :
forall E E' : Ens,
EQ E E' ->
forall a : pi1 E,
EXType (pi1 E') (fun b : pi1 E' => EQ (pi2 E a) (pi2 E' b)).