src/Pure/System/session.scala

   def current_state(): Document.State = global_state.peek()
 
   private case object Interrupt_Prover
   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
 

 
     /* prover results */
 
     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)
     //{{{
     {