src/Pure/PIDE/prover.scala

     receiver(new Prover.Output(XML.Elem(Markup(Markup.SYSTEM, Nil), List(XML.Text(text)))))
   }
 
   private def protocol_output(props: Properties.T, bytes: Bytes)
   {
-    receiver(new Prover.Protocol_Output(props, bytes))
+    receiver(new Prover.Protocol_Output(xml_cache.props(props), bytes))
   }
 
   private def output(kind: String, props: Properties.T, body: XML.Body)
   {
     val main = XML.Elem(Markup(kind, props), Protocol_Message.clean_reports(body))