diff -r c81f8b2967e1 -r 9f03b5f847cd src/HOL/Set.thy
--- a/src/HOL/Set.thy	Thu Jan 29 22:27:07 2009 +0100
+++ b/src/HOL/Set.thy	Thu Jan 29 22:28:03 2009 +0100
@@ -885,6 +885,10 @@
     "A = B ==> (!!x. x:B ==> C x = D x) ==> (UN x:A. C x) = (UN x:B. D x)"
   by (simp add: UNION_def)
 
+lemma strong_UN_cong:
+    "A = B ==> (!!x. x:B =simp=> C x = D x) ==> (UN x:A. C x) = (UN x:B. D x)"
+  by (simp add: UNION_def simp_implies_def)
+
 
 subsubsection {* Intersections of families *}