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-(************************************************************************)
-(* v * The Coq Proof Assistant / The Coq Development Team *)
-(* <O___,, * INRIA - CNRS - LIX - LRI - PPS - Copyright 1999-2016 *)
-(* \VV/ **************************************************************)
-(* // * This file is distributed under the terms of the *)
-(* * GNU Lesser General Public License Version 2.1 *)
-(************************************************************************)
-
-(** This file formalizes Berardi's paradox which says that in
- the calculus of constructions, excluded middle (EM) and axiom of
- choice (AC) imply proof irrelevance (PI).
- Here, the axiom of choice is not necessary because of the use
- of inductive types.
-<<
-@article{Barbanera-Berardi:JFP96,
- author = {F. Barbanera and S. Berardi},
- title = {Proof-irrelevance out of Excluded-middle and Choice
- in the Calculus of Constructions},
- journal = {Journal of Functional Programming},
- year = {1996},
- volume = {6},
- number = {3},
- pages = {519-525}
-}
->> *)
-
-Set Implicit Arguments.
-
-Section Berardis_paradox.
-
-(** Excluded middle *)
-Hypothesis EM : forall P:Prop, P \/ ~ P.
-
-(** Conditional on any proposition. *)
-Definition IFProp (P B:Prop) (e1 e2:P) :=
- match EM B with
- | or_introl _ => e1
- | or_intror _ => e2
- end.
-
-(** Axiom of choice applied to disjunction.
- Provable in Coq because of dependent elimination. *)
-Lemma AC_IF :
- forall (P B:Prop) (e1 e2:P) (Q:P -> Prop),
- (B -> Q e1) -> (~ B -> Q e2) -> Q (IFProp B e1 e2).
-Proof.
-intros P B e1 e2 Q p1 p2.
-unfold IFProp.
-case (EM B); assumption.
-Qed.
-
-
-(** We assume a type with two elements. They play the role of booleans.
- The main theorem under the current assumptions is that [T=F] *)
-Variable Bool : Prop.
-Variable T : Bool.
-Variable F : Bool.
-
-(** The powerset operator *)
-Definition pow (P:Prop) := P -> Bool.
-
-
-(** A piece of theory about retracts *)
-Section Retracts.
-
-Variables A B : Prop.
-
-Record retract : Prop :=
- {i : A -> B; j : B -> A; inv : forall a:A, j (i a) = a}.
-
-Record retract_cond : Prop :=
- {i2 : A -> B; j2 : B -> A; inv2 : retract -> forall a:A, j2 (i2 a) = a}.
-
-
-(** The dependent elimination above implies the axiom of choice: *)
-Lemma AC : forall r:retract_cond, retract -> forall a:A, j2 r (i2 r a) = a.
-Proof.
-intros r.
-case r; simpl.
-trivial.
-Qed.
-
-End Retracts.
-
-(** This lemma is basically a commutation of implication and existential
- quantification: (EX x | A -> P(x)) <=> (A -> EX x | P(x))
- which is provable in classical logic ( => is already provable in
- intuitionnistic logic). *)
-
-Lemma L1 : forall A B:Prop, retract_cond (pow A) (pow B).
-Proof.
-intros A B.
-destruct (EM (retract (pow A) (pow B))) as [(f0,g0,e) | hf].
- exists f0 g0; trivial.
- exists (fun (x:pow A) (y:B) => F) (fun (x:pow B) (y:A) => F); intros;
- destruct hf; auto.
-Qed.
-
-
-(** The paradoxical set *)
-Definition U := forall P:Prop, pow P.
-
-(** Bijection between [U] and [(pow U)] *)
-Definition f (u:U) : pow U := u U.
-
-Definition g (h:pow U) : U :=
- fun X => let lX := j2 (L1 X U) in let rU := i2 (L1 U U) in lX (rU h).
-
-(** We deduce that the powerset of [U] is a retract of [U].
- This lemma is stated in Berardi's article, but is not used
- afterwards. *)
-Lemma retract_pow_U_U : retract (pow U) U.
-Proof.
-exists g f.
-intro a.
-unfold f, g; simpl.
-apply AC.
-exists (fun x:pow U => x) (fun x:pow U => x).
-trivial.
-Qed.
-
-(** Encoding of Russel's paradox *)
-
-(** The boolean negation. *)
-Definition Not_b (b:Bool) := IFProp (b = T) F T.
-
-(** the set of elements not belonging to itself *)
-Definition R : U := g (fun u:U => Not_b (u U u)).
-
-
-Lemma not_has_fixpoint : R R = Not_b (R R).
-Proof.
-unfold R at 1.
-unfold g.
-rewrite AC with (r := L1 U U) (a := fun u:U => Not_b (u U u)).
-trivial.
-exists (fun x:pow U => x) (fun x:pow U => x); trivial.
-Qed.
-
-
-Theorem classical_proof_irrelevence : T = F.
-Proof.
-generalize not_has_fixpoint.
-unfold Not_b.
-apply AC_IF.
-intros is_true is_false.
-elim is_true; elim is_false; trivial.
-
-intros not_true is_true.
-elim not_true; trivial.
-Qed.
-
-End Berardis_paradox.