--- a/ProgTutorial/Base.thy	Fri Oct 02 15:38:14 2009 +0200
+++ b/ProgTutorial/Base.thy	Sat Oct 03 13:01:39 2009 +0200
@@ -17,7 +17,7 @@
 
 ML {*
 (* FIXME ref *)
-val file_name = ref (NONE : string option)
+val file_name = Unsynchronized.ref (NONE : string option)
 
 fun write_file txt =
   case !file_name of