src/Pure/System/progress.scala

   base_progress: Progress,
   input_messages: Boolean = false,
   output_stopped: Boolean = false,
   kind: String = "progress",
   hostname: String = Isabelle_System.hostname(),
-  context_uuid: String = UUID.random().toString,
+  context_uuid: String = UUID.random_string(),
   timeout: Option[Time] = None)
 extends Progress {
   database_progress =>
 
   override def now(): Date = db.now()

   private def init(): Unit = synchronized {
     Progress.private_data.transaction_lock(db, create = true, label = "Database_Progress.init") {
       Progress.private_data.read_progress_context(db, context_uuid) match {
         case Some(context) =>
           _context = context
-          _agent_uuid = UUID.random().toString
+          _agent_uuid = UUID.random_string()
         case None =>
           _context = Progress.private_data.next_progress_context(db)
           _agent_uuid = context_uuid
           db.execute_statement(Progress.private_data.Base.table.insert(), { stmt =>
             stmt.string(1) = context_uuid