diff -r 869b5ea478b0 -r f36cb6f233bd src/Pure/System/session.scala
--- a/src/Pure/System/session.scala	Wed Jan 12 16:04:47 2011 -0800
+++ b/src/Pure/System/session.scala	Thu Jan 13 17:27:52 2011 +0100
@@ -136,6 +136,8 @@
   private case class Edit_Version(edits: List[Document.Edit_Text])
   private case class Start(timeout: Time, args: List[String])
 
+  var verbose: Boolean = false
+
   private val (_, session_actor) = Simple_Thread.actor("session_actor", daemon = true)
   {
     var prover: Isabelle_Process with Isar_Document = null
@@ -188,7 +190,8 @@
 
     def bad_result(result: Isabelle_Process.Result)
     {
-      System.err.println("Ignoring prover result: " + result.message.toString)
+      if (verbose)
+        System.err.println("Ignoring prover result: " + result.message.toString)
     }
 
     def handle_result(result: Isabelle_Process.Result)