Proof @FunctionalExtensionality.functional_extensionality.
(** For compatibility with earlier developments, [extensionality] is an alias for [functional_extensionality]. *)

(* Specifications: *)
Lemma extensionality:
forall {A B: Type} (f g : A -> B),  (forall x, f x = g x) -> f = g.
Lemma extensionality:
forall {A B: Type} (f g : A -> B),  (forall x, f x = g x) -> f = g.