X-Git-Url: http://matita.cs.unibo.it/gitweb/?a=blobdiff_plain;f=helm%2Fsoftware%2Fmatita%2Flibrary%2Fdidactic%2Fexercises%2Fnatural_deduction_fst_order.ma;h=2c796236ddfc022e9829170cbbf1ae27927be00e;hb=f524a0d716de2bdc0874aace8f82f6289034eccf;hp=9a865597b02879bc45ddc827827155693faef0ee;hpb=54e651eaa3cbcc16cfdf3fcc8b53482bffc78b76;p=helm.git

diff --git a/helm/software/matita/library/didactic/exercises/natural_deduction_fst_order.ma b/helm/software/matita/library/didactic/exercises/natural_deduction_fst_order.ma
index 9a865597b..2c796236d 100644
--- a/helm/software/matita/library/didactic/exercises/natural_deduction_fst_order.ma
+++ b/helm/software/matita/library/didactic/exercises/natural_deduction_fst_order.ma
@@ -1,3 +1,17 @@
+(**************************************************************************)
+(*       ___	                                                          *)
+(*      ||M||                                                             *)
+(*      ||A||       A project by Andrea Asperti                           *)
+(*      ||T||                                                             *)
+(*      ||I||       Developers:                                           *)
+(*      ||T||       A.Asperti, C.Sacerdoti Coen,                          *)
+(*      ||A||       E.Tassi, S.Zacchiroli                                 *)
+(*      \   /                                                             *)
+(*       \ /        This file is distributed under the terms of the       *)
+(*        v         GNU Lesser General Public License Version 2.1         *)
+(*                                                                        *)
+(**************************************************************************)
+
 (* Esercizio 0
    ===========
 
@@ -193,7 +207,7 @@ apply rule (prove (¬(∀x.P x) ⇒ ∃x.¬ P x));
 apply rule (⇒_i [h1] (∃x.¬ P x));
 apply rule (RAA [h2] (⊥));
 apply rule (¬_e (¬(∀x.P x)) (∀x.P x));
-	[ apply rule (discharge [h1]);
+	[ apply rule (discharge [h2]);
 	| apply rule (∀_i {y} (P y));
     apply rule (RAA [h3] (⊥));
     apply rule (¬_e (¬∃x.¬ P x) (∃x.¬ P x));