clarified signature;

--- a/src/Pure/PIDE/headless.scala	Fri Mar 27 13:02:56 2020 +0100
+++ b/src/Pure/PIDE/headless.scala	Fri Mar 27 13:04:15 2020 +0100
@@ -562,6 +562,8 @@
   {
     resources =>
 
+    val store: Sessions.Store = Sessions.store(options)
+
 
     /* session */
 
@@ -584,7 +586,6 @@
       session.phase_changed += session_phase
 
       progress.echo("Starting session " + session_base_info.session + " ...")
-      val store = Sessions.store(options)
       Isabelle_Process(session, options, session_base_info.sessions_structure, store,
         logic = session_base_info.session, modes = print_mode)