diff -r fb9f51ba1bbc -r 28496da3bec2 src/Pure/General/markup.scala
--- a/src/Pure/General/markup.scala	Mon Aug 30 11:17:05 2010 +0200
+++ b/src/Pure/General/markup.scala	Mon Aug 30 11:35:17 2010 +0200
@@ -226,6 +226,7 @@
   /* messages */
 
   val PID = "pid"
+  val SERIAL = "serial"
 
   val MESSAGE = "message"
   val CLASS = "class"