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(** This file provides classical logic and unique choice; this is weaker than providing iota operator and classical logic as the definite descriptions provided by the axiom of unique choice can be used only in a propositional context (especially, they cannot be used to build functions outside the scope of a theorem proof) *)
(** Classical logic and unique choice, as shown in [[ChicliPottierSimpson02]], implies the double-negation of excluded-middle in [Set], hence it implies a strongly classical world. Especially it conflicts with the impredicativity of [Set].  [[ChicliPottierSimpson02]] Laurent Chicli, Loïc Pottier, Carlos Simpson, Mathematical Quotients and Quotient Types in Coq, Proceedings of TYPES 2002, Lecture Notes in Computer Science 2646, Springer Verlag.  *)
Require Export Classical.
Axiom
dependent_unique_choice :
forall (A:Type) (B:A -> Type) (R:forall x:A, B x -> Prop),
(forall x : A, exists! y : B x, R x y) ->
(exists f : (forall x:A, B x), forall x:A, R x (f x)).
(** Unique choice reifies functional relations into functions *)
(** The following proof comes from [[ChicliPottierSimpson02]] *)

Theorem classic_set_in_prop_context :
forall C:Prop, ((forall P:Prop, {P} + {~ P}) -> C) -> C.