diff -r f8b1a75dbea7 -r a3177042863d src/Pure/Thy/sessions.scala
--- a/src/Pure/Thy/sessions.scala	Fri Dec 16 16:00:56 2022 +0100
+++ b/src/Pure/Thy/sessions.scala	Fri Dec 16 17:02:10 2022 +0100
@@ -56,6 +56,10 @@
 
   /* base info */
 
+  object Base {
+    val bootstrap: Base = Base(overall_syntax = Thy_Header.bootstrap_syntax)
+  }
+
   sealed case class Base(
     session_name: String = "",
     session_pos: Position.T = Position.none,
@@ -97,8 +101,6 @@
       nodes(name).syntax orElse loaded_theory_syntax(name) getOrElse overall_syntax
   }
 
-  val bootstrap_base: Base = Base(overall_syntax = Thy_Header.bootstrap_syntax)
-
   sealed case class Base_Info(
     base: Base,
     sessions_structure: Structure = Structure.empty,
@@ -257,7 +259,7 @@
 
     val session_bases =
       sessions_structure.imports_topological_order.foldLeft(
-          Map(Sessions.bootstrap_base.session_entry)) {
+          Map(Sessions.Base.bootstrap.session_entry)) {
         case (session_bases, session_name) =>
           progress.expose_interrupt()