diff -r 5fab62ae3532 -r ac6648c0c0fb src/Pure/System/session.scala
--- a/src/Pure/System/session.scala	Fri Aug 09 10:41:17 2013 +0200
+++ b/src/Pure/System/session.scala	Fri Aug 09 11:18:36 2013 +0200
@@ -238,6 +238,7 @@
   /* actor messages */
 
   private case class Start(args: List[String])
+  private case class Cancel_Exec(exec_id: Document_ID.Exec)
   private case class Change(
     doc_edits: List[Document.Edit_Command],
     previous: Document.Version,
@@ -506,6 +507,9 @@
           global_options.event(Session.Global_Options(options))
           reply(())
 
+        case Cancel_Exec(exec_id) if prover.isDefined =>
+          prover.get.cancel_exec(exec_id)
+
         case Session.Raw_Edits(edits) if prover.isDefined =>
           handle_raw_edits(edits)
           reply(())
@@ -552,6 +556,8 @@
     session_actor !? Stop
   }
 
+  def cancel_exec(exec_id: Document_ID.Exec) { session_actor ! Cancel_Exec(exec_id) }
+
   def update(edits: List[Document.Edit_Text])
   { if (!edits.isEmpty) session_actor !? Session.Raw_Edits(edits) }