fixed type error;

--- a/src/Pure/Concurrent/lazy_sequential.ML	Thu Jan 07 17:41:06 2010 +0100
+++ b/src/Pure/Concurrent/lazy_sequential.ML	Fri Jan 08 11:07:53 2010 +0100
@@ -37,7 +37,7 @@
     val _ =
       (case result of
         Exn.Exn Exn.Interrupt => ()
-      | _ => r := result);
+      | _ => r := Result result);
   in result end;
 
 fun force r = Exn.release (force_result r);