--- a/ProgTutorial/Solutions.thy	Fri Oct 02 15:38:14 2009 +0200
+++ b/ProgTutorial/Solutions.thy	Sat Oct 03 13:01:39 2009 +0200
@@ -273,7 +273,7 @@
 
 ML{*fun term_tree n =
 let
-  val count = ref 0; 
+  val count = Unsynchronized.ref 0; 
 
   fun term_tree_aux n =
     case n of