src/Pure/Thy/export_theory.scala

       axioms.iterator.map(_.no_content) ++
       thms.iterator.map(_.no_content) ++
       classes.iterator.map(_.no_content) ++
       locales.iterator.map(_.no_content) ++
       locale_dependencies.iterator.map(_.no_content)
+
+    lazy val entity_serials: Set[Long] =
+      entity_iterator.map(_.serial).toSet
 
     def cache(cache: Term.Cache): Theory =
       Theory(cache.string(name),
         parents.map(cache.string),
         types.map(_.cache(cache)),