diff -r e1bd8a54c40f -r d7728f65b353 src/HOL/Wellfounded.thy
--- a/src/HOL/Wellfounded.thy	Mon Sep 13 08:43:48 2010 +0200
+++ b/src/HOL/Wellfounded.thy	Mon Sep 13 11:13:15 2010 +0200
@@ -781,7 +781,7 @@
 
           let ?N1 = "{ n \<in> N. (n, a) \<in> r }"
           let ?N2 = "{ n \<in> N. (n, a) \<notin> r }"
-          have N: "?N1 \<union> ?N2 = N" by (rule set_ext) auto
+          have N: "?N1 \<union> ?N2 = N" by (rule set_eqI) auto
           from Nless have "finite N" by (auto elim: max_ext.cases)
           then have finites: "finite ?N1" "finite ?N2" by auto