1 files changed, 90 insertions, 0 deletions
diff --git a/theories/Logic/ClassicalEpsilon.v b/theories/Logic/ClassicalEpsilon.v
new file mode 100644
index 00000000..b7293bec
--- /dev/null
+++ b/theories/Logic/ClassicalEpsilon.v
@@ -0,0 +1,90 @@
+(************************************************************************)
+(*  v      *   The Coq Proof Assistant  /  The Coq Development Team     *)
+(* <O___,, * CNRS-Ecole Polytechnique-INRIA Futurs-Universite Paris Sud *)
+(*   \VV/  **************************************************************)
+(*    //   *      This file is distributed under the terms of the       *)
+(*         *       GNU Lesser General Public License Version 2.1        *)
+(************************************************************************)
+
+(*i $Id: ClassicalEpsilon.v 8933 2006-06-09 14:08:38Z herbelin $ i*)
+
+(** *** This file provides classical logic and indefinite description
+    (Hilbert's epsilon operator) *)
+
+(** Classical epsilon's operator (i.e. indefinite description) implies
+    excluded-middle in [Set] and leads to a classical world populated
+    with non computable functions. It conflicts with the
+    impredicativity of [Set] *)
+
+Require Export Classical.
+Require Import ChoiceFacts.
+
+Set Implicit Arguments.
+
+Notation Local "'inhabited' A" := A (at level 200, only parsing).
+
+Axiom constructive_indefinite_description :
+  forall (A : Type) (P : A->Prop), 
+  (ex P) -> { x : A | P x }.
+
+Lemma constructive_definite_description :
+  forall (A : Type) (P : A->Prop), 
+  (exists! x : A, P x) -> { x : A | P x }.
+Proof.
+intros; apply constructive_indefinite_description; firstorder.
+Qed.
+
+Theorem excluded_middle_informative : forall P:Prop, {P} + {~ P}.
+Proof.
+apply 
+  (constructive_definite_descr_excluded_middle 
+   constructive_definite_description classic).
+Qed.
+
+Theorem classical_indefinite_description : 
+  forall (A : Type) (P : A->Prop), inhabited A ->
+  { x : A | ex P -> P x }.
+Proof.
+intros A P i.
+destruct (excluded_middle_informative (exists x, P x)) as [Hex|HnonP].
+  apply constructive_indefinite_description with (P:= fun x => ex P -> P x).
+  destruct Hex as (x,Hx).
+    exists x; intros _; exact Hx.
+    firstorder.
+Qed.
+
+(** Hilbert's epsilon operator *)
+
+Definition epsilon (A : Type) (i:inhabited A) (P : A->Prop) : A
+  := proj1_sig (classical_indefinite_description P i).
+
+Definition epsilon_spec (A : Type) (i:inhabited A) (P : A->Prop) : 
+  (ex P) -> P (epsilon i P)
+  := proj2_sig (classical_indefinite_description P i).
+
+Opaque epsilon.
+
+(** Open question: is classical_indefinite_description constructively
+    provable from [relational_choice] and
+    [constructive_definite_description] (at least, using the fact that
+    [functional_choice] is provable from [relational_choice] and
+    [unique_choice], we know that the double negation of
+    [classical_indefinite_description] is provable (see
+    [relative_non_contradiction_of_indefinite_desc]). *)
+
+(** Remark: we use [ex P] rather than [exists x, P x] (which is [ex
+    (fun x => P x)] to ease unification *)
+
+(** *** Weaker lemmas (compatibility lemmas) *)
+
+Theorem choice :
+ forall (A B : Type) (R : A->B->Prop),
+   (forall x : A, exists y : B, R x y) ->
+   (exists f : A->B, forall x : A, R x (f x)).
+Proof.
+intros A B R H.
+exists (fun x => proj1_sig (constructive_indefinite_description (H x))).
+intro x.
+apply (proj2_sig (constructive_indefinite_description  (H x))).
+Qed.
+