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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.
simple induction E; intros A f r; simple induction E'; intros A' f' r';
simpl in |- *; simple induction 1; intros e1 e2 a.
apply e1.
Defined.
simple induction E; simpl in |- *.
intros A f r.
simple induction 1; simpl in |- *.
intros.
exists x; auto with zfc.
Qed.
(* The union axioms *)
simple induction E; intros A f r.
intros.
simpl in |- *.
elim (IN_EXType (sup A f) E' H).
intros x e.
cut (EQ (pi2 (sup A f) x) E'); auto with zfc.
intros e1.
cut (IN E'' (pi2 (sup A f) x)).
intros i1.
elim (IN_EXType _ _ i1).
intros x0 e2.
simpl in x0.
exists (dep_i A (fun x : A => pi1 (f x)) x x0).
simpl in |- *.
exact e2.
apply IN_sound_right with E'; auto with zfc.
Qed.
unfold INC in |- *; simple induction E; intros A f r; unfold Union in |- *.
intros E' i E'' i'; simpl in |- *; elim (IN_EXType (sup A f) E' i).
intros a e; simpl in a; simpl in e.
elim (IN_EXType E' E'' i').
cut (IN E'' (f a)).
intros i'' a' e''; elim (IN_EXType _ _ i''); simpl in |- *; intros aa ee.
exists (dep_i A (fun x : A => pi1 (f x)) a aa); auto with zfc.
apply IN_sound_right with E'; auto with zfc.
Qed.
simple induction E; unfold Union in |- *; simpl in |- *; intros A f r.
simple induction 1.
simple induction x.
intros a b; simpl in |- *.
intros.
exists (f a).
split.
exists a; auto with zfc.
apply IN_sound_left with (pi2 (f a) b); auto with zfc.
simpl in |- *.
generalize b; elim (f a); simpl in |- *.
intros.
exists b0; auto with zfc.
Qed.
(* extentionality of union  *)
unfold Union in |- *; simple induction E; intros A f r; simple induction E';
intros A' f' r'; simpl in |- *; simple induction 1;
intros e1 e2; split.
intros x; elim x; intros a aa; elim (e1 a); intros a' ea.
elim (EQ_EXType (f a) (f' a') ea aa); intros aa' eaa.
exists (dep_i A' (fun x : A' => pi1 (f' x)) a' aa'); simpl in |- *;
auto with zfc.
intros c'; elim c'; intros a' aa'; elim (e2 a'); intros a ea.
cut (EQ (f' a') (f a)).
2: auto with zfc.
intros ea'; elim (EQ_EXType (f' a') (f a) ea' aa'); intros aa eaa.
exists (dep_i A (fun x : A => pi1 (f x)) a aa); auto with zfc.
Qed.
(* The union construction is monotone w.r.t. inclusion   *)
unfold INC in |- *; intros E E' IEE E'' IEE''.
elim (Union_IN E E'').
intros E'''; simple induction 1; intros I1 I2.
apply IN_Union with E'''; auto with zfc.
auto with zfc.
Qed.
(* The  Intersection set   *)
Definition Inter (E : Ens) : Ens :=
match E with
| sup A f =>
sup _
(fun
c : depprod _
(fun a : A =>
depprod _
(fun b : pi1 (f a) =>
forall x : A, IN (pi2 (f a) b) (f x))) =>
match c with
| dep_i a (dep_i b p) => pi2 (f a) b
end)
end.
(* the axioms *)
simple induction E; intros A f r; simpl in |- *; intros E'.
simple induction 1; intros c; elim c; intros a ca; elim ca; intros aa paa;
simpl in |- *.
intros e E'' e''.
elim e''; intros a1 ea1.
apply IN_sound_right with (f a1); auto with zfc.
apply IN_sound_left with (pi2 (f a) aa); auto with zfc.
Qed.
simple induction E; intros A f r.
intros E' E'' i H.
elim (IN_EXType (sup A f) E'' i).
intros a e; simpl in a.
simpl in e.
cut (IN E' E''); auto with zfc.
intros i'.
cut (IN E' (f a)); auto with zfc.
intros i0.
elim (IN_EXType (f a) E' i0).
intros b e'.
simpl in |- *.
cut (forall x : A, IN (pi2 (f a) b) (f x)).
intros.
exists
(dep_i A
(fun a : A =>
depprod (pi1 (f a))
(fun b : pi1 (f a) => forall x : A, IN (pi2 (f a) b) (f x))) a
(dep_i (pi1 (f a))
(fun b : pi1 (f a) => forall x : A, IN (pi2 (f a) b) (f x)) b H0)).
simpl in |- *.
auto with zfc.
auto with zfc.
intros.
apply IN_sound_left with E'.
auto with zfc.
apply H.
auto with zfc.
simpl in |- *.
exists x; auto with zfc.
apply IN_sound_right with E''; auto with zfc.
Qed.
(*  The powerset and its axioms   *)
Definition Power (E : Ens) : Ens :=
match E with
| sup A f =>
sup _
(fun P : A -> Prop =>
sup _
(fun c : depprod A (fun a : A => P a) =>
match c with
| dep_i a p => f a
end))
end.
simple induction E.
intros A f r; unfold INC in |- *; simpl in |- *.
intros E'; simple induction 1; intros P.
elim E'; simpl in |- *.
intros A' f' r'.
simple induction 1; intros HA HB.
intros E''; simple induction 1; intros a' e.
elim (HA a').
simple induction x; intros a p.
intros; exists a.
auto with zfc.
apply EQ_tran with (f' a'); auto with zfc.
Qed.
simple induction E; intros A f r; unfold INC in |- *; simpl in |- *.
simple induction E'; intros A' f' r' i.
exists (fun a : A => IN (f a) (sup A' f')).
simpl in |- *.
split.
intros.
elim (i (f' x)); auto with zfc.
intros a e.
cut (EQ (f a) (f' x)); auto with zfc.
intros e1.
exists
(dep_i A (fun a : A => EXType A' (fun y : A' => EQ (f a) (f' y))) a
(EXTypei A' (fun y : A' => EQ (f a) (f' y)) x e1)).
simpl in |- *.
auto with zfc.
auto with zfc.
simpl in |- *.
exists x; auto with zfc.
simple induction y; simpl in |- *.
simple induction 1; intros.
exists x0; auto with zfc.
Qed.
intros.
unfold INC in |- *; intros.
apply INC_IN_Power.
cut (INC E0 E).
unfold INC in |- *; unfold INC in H; intros; auto with zfc.
apply IN_Power_INC; auto with zfc.
Qed.
intros E E' e.
apply INC_EQ; unfold INC in |- *.
intros A i.
cut (INC A E').
intros; apply INC_IN_Power; assumption.
cut (INC A E); intros.
apply INC_sound_right with E; auto with zfc.
apply IN_Power_INC; assumption.
intros A i.
cut (INC A E).
intros; apply INC_IN_Power; assumption.
cut (INC A E'); intros.
apply INC_sound_right with E'; auto with zfc.
apply IN_Power_INC; assumption.
Qed.
(* small lemma *)
intros E e; cut False.
simple induction 1.
cut (IN E Vide).
simpl in |- *; simple induction 1; intros xx; elim xx; simple induction 1.
apply IN_sound_right with (Sing E); auto with zfc.
Qed.
intros E e; cut False.
simple induction 1.
cut (IN E Vide).
simpl in |- *; simple induction 1; intros xx; elim xx; simple induction 1.
apply IN_sound_right with (Sing E); auto with zfc.
Qed.
(* The cartesian product and its properties *)
(* This definition of the ordered pair is slightly different from *)
(* the usual one, since we want it to work in an intuisionistic   *)
(* setting. Works the same, neitherless. The soundness proofs are *)
(* unpleasant.                                                    *)
Definition Couple (E E' : Ens) := Paire (Sing E) (Paire Vide (Sing E')).
unfold Couple in |- *; simpl in |- *.
simple induction 1.
intros HA HB; elim (HA true).
intros x; elim x; simpl in |- *; simple induction 1; intros H3 H4;
elim (H3 true); simpl in |- *; intros xx; elim xx;
simpl in |- *; auto with zfc.
elim (H4 false); simpl in |- *.
simple induction x0; simpl in |- *.
intros.
cut (EQ (Sing B') Vide).
simpl in |- *.
simple induction 1.
intros yy; elim (yy true).
simple induction x1.
apply EQ_tran with A; auto with zfc.
intros; cut (EQ (Sing B') Vide).
simpl in |- *.
simple induction 1.
intros yy; elim (yy true).
simple induction x1.
apply EQ_tran with A; auto with zfc.
intros yy.
elim (HB true); simpl in |- *.
simple induction x0.
change (EQ (Sing A) (Sing B) -> EQ A B) in |- *; intros EE.
apply IN_Sing_EQ.
apply IN_sound_right with (Sing A); auto with zfc.
change (EQ (Paire Vide (Sing A')) (Sing B) -> EQ A B) in |- *.
intros zz.
elimtype F.
apply (not_EQ_Sing_Vide A').
apply EQ_tran with B.
apply IN_Sing_EQ.
apply IN_sound_right with (Paire Vide (Sing A')); auto with zfc.
apply EQ_sym; apply IN_Sing_EQ;
apply IN_sound_right with (Paire Vide (Sing A'));
auto with zfc.
Qed.
unfold Couple in |- *; simpl in |- *.
simple induction 1; intros H1 H2.
elim (H1 false).
intros bb1; elim bb1.
intros HF.
change (EQ (Paire Vide (Sing A')) (Sing B)) in HF.
cut F.
simple induction 1.
apply (not_EQ_Vide_Sing A').
apply EQ_tran with B.
apply IN_Sing_EQ; apply IN_sound_right with (Paire Vide (Sing A'));
auto with zfc.
apply EQ_sym; apply IN_Sing_EQ;
apply IN_sound_right with (Paire Vide (Sing A'));
auto with zfc.
change (EQ (Paire Vide (Sing A')) (Paire Vide (Sing B')) -> EQ A' B') in |- *.
intros HP; cut (EQ (Sing A') (Sing B')).
intros; auto with zfc.
cut (IN (Sing A') (Paire Vide (Sing B'))).
intros HI; elim (Paire_IN Vide (Sing B') (Sing A') HI).
intros; cut F.
simple induction 1.
apply not_EQ_Sing_Vide with A'; assumption.
trivial with zfc.
apply IN_sound_right with (Paire Vide (Sing A')); auto with zfc.
Qed.
(* Here we cheat. It is easier to define the cartesian product using    *)
(* the type theoretical product, i.e. we here use non set-theoretical   *)
(* constructions. We could however use the usual definitions.           *)
Definition Prod (E E' : Ens) : Ens :=
match E, E' with
| sup A f, sup A' f' =>
sup _
(fun c : prod_t A A' =>
match c with
| pair_t a a' => Couple (f a) (f' a')
end)
end.
Hint Resolve Paire_sound_left Paire_sound_right: zfc.
unfold Couple in |- *; intros; auto with zfc.
Qed.
unfold Couple in |- *; intros; auto with zfc.
Qed.
simple induction E1; intros A1 f1 r1; simple induction E2; intros A2 f2 r2.
intros E1' E2' i1 i2.
elim (IN_EXType (sup A1 f1) E1').
intros x e1; simpl in x.
elim (IN_EXType (sup A2 f2) E2').
intros x0 e2; simpl in x.
apply IN_sound_left with (Couple (pi2 (sup A1 f1) x) (pi2 (sup A2 f2) x0));
auto with zfc.
apply EQ_tran with (Couple (pi2 (sup A1 f1) x) E2'); auto with zfc.
apply Couple_sound_right.
auto with zfc.
apply Couple_sound_left; auto with zfc.
simpl in |- *.
simpl in |- *.
exists (pair_t _ _ x x0).
simpl in |- *.
split.
simple induction x1; simpl in |- *.
exists true; simpl in |- *.
split.
simple induction x2; simpl in |- *.
exists true; auto with zfc.
exists true; auto with zfc.
simple induction y; exists true; auto with zfc.
exists false; simpl in |- *.
split.
simple induction x2.
exists true; simpl in |- *; auto with zfc.
split.
simple induction x3.
simple induction y.
exists false; auto with zfc.
simple induction y; simpl in |- *.
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.
auto with zfc.
auto with zfc.
Qed.
simple induction E1; intros A1 f1 r1; simple induction E2; intros A2 f2 r2.
intros E1' E2' i.
elim (IN_EXType (Prod (sup A1 f1) (sup A2 f2)) (Couple E1' E2') i).
intros xx; elim xx; intros a1 a2 e.
change (EQ (Couple E1' E2') (Couple (f1 a1) (f2 a2))) in e.
cut (EQ E1' (f1 a1)).
cut (EQ E2' (f2 a2)).
intros e1 e2.
split.
apply IN_sound_left with (f1 a1); auto with zfc; simpl in |- *; exists a1;
auto with zfc.
apply IN_sound_left with (f2 a2); auto with zfc; simpl in |- *; exists a2;
auto with zfc.
apply Couple_inj_right with (A := E1') (B := f1 a1); auto with zfc.
apply Couple_inj_left with E2' (f2 a2); auto with zfc.
Qed.
simple induction E; intros A f r; simple induction E'; intros A' f' r'.
intros; elim (IN_EXType (Prod (sup A f) (sup A' f')) E'').
simple induction x.
intros; exists (f a); exists (f' b); auto with zfc.
auto with zfc.
Qed.
(* Ordinals *)
Definition Succ (E : Ens) := Union (Paire E (Sing E)).
Inductive Ord : Ens -> Prop :=
| Oo : Ord Vide
| So : forall E : Ens, Ord E -> Ord (Succ E)
| Lo : forall E : Ens, (forall e : Ens, IN e E -> Ord e) -> Ord (Union E)
| Eo : forall E E' : Ens, Ord E -> EQ E E' -> Ord E'.
Hint Resolve Oo So Lo: zfc.
Definition Nat : nat -> Ens.
simple induction 1; intros.
exact Vide.
exact (Succ X).
Defined.
simple induction n; simpl in |- *; auto with zfc.
Qed.
Definition Omega : Ens := sup nat Nat.
intros E; unfold Succ in |- *; unfold Sing in |- *;
apply IN_Union with (Paire E E); auto with zfc.
Qed.
unfold INC in |- *; unfold Succ in |- *.
intros.
apply IN_Union with E; auto with zfc.
Qed.
Hint Resolve IN_Succ INC_Succ: zfc.
intros E E' i.
unfold Succ in i.
elim (Union_IN (Paire E (Sing E)) E' i).
intros E1; simple induction 1; intros i1 i2.
elim (Paire_IN E (Sing E) E1 i1).
intros; right; apply IN_sound_right with E1; auto with zfc.
intros; left; cut (IN E' (Sing E)).
auto with zfc.
apply IN_sound_right with E1; auto with zfc.
Qed.
simple induction E; intros A f r i.
cut False.
simple induction 1.
elim (IN_EXType (sup A f) (sup A f) i); intros a e.
simpl in a.
change (EQ (sup A f) (f a)) in e.
elim (r a).
apply IN_sound_right with (sup A f); auto with zfc.
exists a; auto with zfc.
Qed.

Theorem Nat_IN_Omega : forall n : nat, IN (Nat n) Omega.