src/Pure/System/session.scala

   val commands_changed = new Event_Bus[Session.Commands_Changed]
   val phase_changed = new Event_Bus[Session.Phase]
   val syslog_messages = new Event_Bus[Isabelle_Process.Output]
   val raw_output_messages = new Event_Bus[Isabelle_Process.Output]
   val all_messages = new Event_Bus[Isabelle_Process.Message]  // potential bottle-neck
-
+  val trace_events = new Event_Bus[Simplifier_Trace.Event.type]
 
 
   /** buffered command changes (delay_first discipline) **/
 
   //{{{