tuned signature (see also 8342cba8eae8);

--- a/src/Pure/PIDE/resources.scala	Fri Dec 16 15:14:09 2022 +0100
+++ b/src/Pure/PIDE/resources.scala	Fri Dec 16 16:00:56 2022 +0100
@@ -13,8 +13,7 @@
 
 
 object Resources {
-  def empty: Resources =
-    new Resources(Sessions.Structure.empty, Sessions.Structure.empty.bootstrap)
+  def empty: Resources = new Resources(Sessions.Structure.empty, Sessions.bootstrap_base)
 
   def hidden_node(name: Document.Node.Name): Boolean =
     !name.is_theory || name.theory == Sessions.root_name || File_Format.registry.is_theory(name)

--- a/src/Pure/Thy/sessions.scala	Fri Dec 16 15:14:09 2022 +0100
+++ b/src/Pure/Thy/sessions.scala	Fri Dec 16 16:00:56 2022 +0100
@@ -792,8 +792,6 @@
   ) {
     sessions_structure =>
 
-    def bootstrap: Base = Base(overall_syntax = Thy_Header.bootstrap_syntax)
-
     def dest_session_directories: List[(String, String)] =
       for ((file, session) <- session_directories.toList)
         yield (File.standard_path(file), session)