(** ** Logical equivalence **)
Definition iff (A B : Prop) : Prop := and (A -> B) (B -> A).
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(** * Existential quantifiers and Leibniz equality  *)
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(** ** Existential quantification in a given small type **)
Definition ex (X : Typ1) (P : X -> Prop) : Prop :=
forall E : Prop, (forall x : X, P x -> E) -> E.

Lemma ex_intro : forall (X : Typ1) (P : X -> Prop) (x : X), P x -> ex X P.
Proof fun X P x p E f => f x p.