X-Git-Url: http://matita.cs.unibo.it/gitweb/?a=blobdiff_plain;f=helm%2Fmatita%2Flibrary%2Flogic%2Fconnectives.ma;h=4cbea3529a7ec983b39f814f1243f1a5f92568ab;hb=4167cea65ca58897d1a3dbb81ff95de5074700cc;hp=15b1cfe10d59a44ccf2e28a7476c72e8d7d61392;hpb=2a990279a83acb9736bbaaf3d978bc4ae994880b;p=helm.git

diff --git a/helm/matita/library/logic/connectives.ma b/helm/matita/library/logic/connectives.ma
index 15b1cfe10..4cbea3529 100644
--- a/helm/matita/library/logic/connectives.ma
+++ b/helm/matita/library/logic/connectives.ma
@@ -56,8 +56,21 @@ inductive Or (A,B:Prop) : Prop \def
 (*CSC: the URI must disappear: there is a bug now *)
 interpretation "logical or" 'or x y =
   (cic:/matita/logic/connectives/Or.ind#xpointer(1/1) x y).
-   
-definition decidable : Prop \to Prop \def \lambda A:Prop. A \lor \not A.
+
+theorem Or_ind':
+ \forall A,B:Prop.
+  \forall P: A \lor B \to Prop.
+   (\forall p:A. P (or_introl ? ? p)) \to
+   (\forall q:B. P (or_intror ? ? q)) \to
+    \forall p:A \lor B. P p.
+ intros.
+ apply
+  (match p return \lambda p.P p with
+    [(or_introl p) \Rightarrow H p
+    |(or_intror q) \Rightarrow H1 q]).
+qed.
+
+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.