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Axiom fun_extensionality : forall A B (f g : A -> B),
(forall x, f x = g x) -> f = g.
Axiom fun_extensionality_dep : forall A, forall B : (A -> Type), forall (f g : forall x : A, B x),
(forall x, f x = g x) -> f = g.
Hint Resolve fun_extensionality fun_extensionality_dep : subtac.
Require Import Coq.subtac.Utils.
Require Import Coq.subtac.FixSub.
Lemma fix_sub_eq_ext :
forall (A : Set) (R : A -> A -> Prop) (Rwf : well_founded R)
(P : A -> Set)
(F_sub : forall x : A, (forall {y : A | R y x}, P (`y)) -> P x),
forall x : A,
Fix_sub A R Rwf P F_sub x =
F_sub x (fun {y : A | R y x}=> Fix A R Rwf P F_sub (`y)).
Proof.
intros ; apply Fix_eq ; auto.
intros.
assert(f = g).
apply (fun_extensionality_dep _ _ _ _ H).
rewrite H0 ; auto.
Qed.
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