diff -r 985c3a64748c -r 8cf14d4ebec4 src/Pure/Thy/export_theory.scala
--- a/src/Pure/Thy/export_theory.scala	Fri Aug 05 13:43:14 2022 +0200
+++ b/src/Pure/Thy/export_theory.scala	Fri Aug 05 14:05:42 2022 +0200
@@ -107,13 +107,9 @@
         (for ((k, xs) <- others.iterator) yield cache.string(k) -> xs.map(_.cache(cache))).toMap)
   }
 
-  def read_theory_parents(provider: Export.Provider, theory_name: String): Option[List[String]] = {
-    if (theory_name == Thy_Header.PURE) Some(Nil)
-    else {
-      provider.focus(theory_name)(Export.THEORY_PREFIX + "parents")
-        .map(entry => split_lines(entry.uncompressed.text))
-    }
-  }
+  def read_theory_parents(provider: Export.Provider, theory_name: String): Option[List[String]] =
+    provider.focus(theory_name)(Export.THEORY_PREFIX + "parents")
+      .map(entry => Library.trim_split_lines(entry.uncompressed.text))
 
   def no_theory: Theory =
     Theory("", Nil, Nil, Nil, Nil, Nil, Nil, Nil, Nil, Nil, Nil, Nil, Nil, Nil, Nil, Map.empty)