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(** * Definitions *)

(** Propositional extensionality *)

Local Notation PropositionalExtensionality :=
  (forall A B : Prop, (A <-> B) -> A = B).(** Provable-proposition extensionality *)

Local Notation ProvablePropositionExtensionality :=
  (forall A:Prop, A -> A = True).(** Refutable-proposition extensionality *)

Local Notation RefutablePropositionExtensionality :=
  (forall A:Prop, ~A -> A = False).(** Predicate extensionality *)

Local Notation PredicateExtensionality :=
  (forall (A:Type) (P Q : A -> Prop), (forall x, P x <-> Q x) -> P = Q).(** Propositional functional extensionality *)

Local Notation PropositionalFunctionalExtensionality :=
  (forall (A:Type) (P Q : A -> Prop), (forall x, P x = Q x) -> P = Q).

Lemma PredExt_imp_PropExt : PredicateExtensionality -> PropositionalExtensionality.