diff -r d1c9bf0f8ae8 -r e66b41782cb5 src/Pure/Concurrent/synchronized_sequential.ML
--- a/src/Pure/Concurrent/synchronized_sequential.ML	Thu Oct 22 09:50:29 2009 +0200
+++ b/src/Pure/Concurrent/synchronized_sequential.ML	Thu Oct 22 15:19:44 2009 +0200
@@ -20,8 +20,13 @@
 
 fun guarded_access var f = the (timed_access var (K NONE) f);
 
+fun readonly_access var f =
+  guarded_access var (fn x => (case f x of NONE => NONE | SOME y => SOME (y, x)));
+
 fun change_result var f = guarded_access var (SOME o f);
 fun change var f = change_result var (fn x => ((), f x));
 
+val assign = change;
+
 end;
 end;