src/Pure/System/progress.scala

--- a/src/Pure/System/progress.scala	Tue May 29 22:29:32 2018 +0200
+++ b/src/Pure/System/progress.scala	Wed May 30 14:34:43 2018 +0200
@@ -51,7 +51,10 @@
   }
 
   override def theory(session: String, theory: String): Unit =
-    if (verbose) echo(session + ": theory " + theory)
+    if (verbose) {
+      if (session == "") echo("theory " + theory)
+      else echo(session + ": theory " + theory)
+    }
 
   @volatile private var is_stopped = false
   override def interrupt_handler[A](e: => A): A =