(** Church's iota operator *)
Definition iota (A : Type) (i:inhabited A) (P : A->Prop) : A
:= proj1_sig (classical_definite_description P i).
Definition iota_spec (A : Type) (i:inhabited A) (P : A->Prop) :
(exists! x:A, P x) -> P (iota i P)
:= proj2_sig (classical_definite_description P i).

(* Specifications: *)
(** Axiom of unique "choice" (functional reification of functional relations) *)
Theorem 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)).
Theorem unique_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)).