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(** This module states the functional form of the axiom of choice over setoids, commonly called extensional axiom of choice [[Carlström04]], [[Martin-Löf05]]. This is obtained by a decomposition of the axiom into the following components:  - classical logic - relational axiom of choice - axiom of unique choice - a limited form of functional extensionality  Among other results, it entails: - proof irrelevance - choice of a representative in equivalence classes  [[Carlström04]] Jesper Carlström, EM + Ext_ + AC_int is equivalent to AC_ext, Mathematical Logic Quaterly, vol 50(3), pp 236-240, 2004.  [[Martin-Löf05] Per Martin-Löf, 100 years of Zermelo’s axiom of choice: what was the problem with it?, lecture notes for KTH/SU colloquium, 2005.  *)
Require Export ClassicalChoice. (* classical logic, relational choice, unique choice *)
Require Export ExtensionalFunctionRepresentative.

Theorem setoid_choice :
forall A B,
forall R : A -> A -> Prop,
forall T : A -> B -> Prop,
Equivalence R ->
(forall x x' y, R x x' -> T x y -> T x' y) ->
(forall x, exists y, T x y) ->
exists f : A -> B, forall x : A, T x (f x) /\ (forall x' : A, R x x' -> f x = f x').