default "absurd" cic:/matita/logic/connectives/absurd.con.
+theorem not_to_not : \forall A,B:Prop. (A → B) \to ¬B →¬A.
+intros.unfold.intro.apply H1.apply (H H2).
+qed.
+
+default "absurd" cic:/matita/logic/connectives/absurd.con.
+
inductive And (A,B:Prop) : Prop \def
conj : A \to B \to (And A B).
inductive ex (A:Type) (P:A \to Prop) : Prop \def
ex_intro: \forall x:A. P x \to ex A P.
-interpretation "exists" 'exists \eta.x = (ex _ x).
+interpretation "exists" 'exists x = (ex ? x).
inductive ex2 (A:Type) (P,Q:A \to Prop) : Prop \def
ex_intro2: \forall x:A. P x \to Q x \to ex2 A P Q.