diff -r a20631db9c8a -r 8517172b9626 src/Pure/System/session.scala
--- a/src/Pure/System/session.scala	Mon Jul 29 12:50:16 2013 +0200
+++ b/src/Pure/System/session.scala	Mon Jul 29 13:00:36 2013 +0200
@@ -238,7 +238,6 @@
   /* actor messages */
 
   private case class Start(args: List[String])
-  private case object Cancel_Execution
   private case class Change(
     doc_edits: List[Document.Edit_Command],
     previous: Document.Version,
@@ -504,9 +503,6 @@
           global_options.event(Session.Global_Options(options))
           reply(())
 
-        case Cancel_Execution if prover.isDefined =>
-          prover.get.cancel_execution()
-
         case Session.Raw_Edits(edits) if prover.isDefined =>
           handle_raw_edits(edits)
           reply(())
@@ -553,8 +549,6 @@
     session_actor !? Stop
   }
 
-  def cancel_execution() { session_actor ! Cancel_Execution }
-
   def update(edits: List[Document.Edit_Text])
   { if (!edits.isEmpty) session_actor !? Session.Raw_Edits(edits) }