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).