--- a/ProgTutorial/General.thy	Fri Oct 02 15:38:14 2009 +0200
+++ b/ProgTutorial/General.thy	Sat Oct 03 13:01:39 2009 +0200
@@ -1000,7 +1000,7 @@
   of theorems.
 *}
 
-ML{*val my_thms = ref ([] : thm list)*}
+ML{*val my_thms = Unsynchronized.ref ([] : thm list)*}
 
 text {* 
   The purpose of this reference is to store a list of theorems.