not_to_not

[helm.git] / helm / software / matita / library / logic / connectives.ma

diff --git a/helm/software/matita/library/logic/connectives.ma b/helm/software/matita/library/logic/connectives.ma
index 832d1a531fee7a18ee64c1de57997af27a867f15..76d0323166ad947578481e7c06bb4d8d8b3cc41a 100644 (file)
--- a/helm/software/matita/library/logic/connectives.ma
+++ b/helm/software/matita/library/logic/connectives.ma
@@ -32,6 +32,12 @@ qed.
 
 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).
 
@@ -69,7 +75,7 @@ definition decidable : Prop \to Prop \def \lambda A:Prop. A \lor \lnot A.
 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.