diff -r ac1da69fbc5a -r c9c1c8b28a62 src/Pure/context.ML
--- a/src/Pure/context.ML	Thu Oct 16 22:44:25 2008 +0200
+++ b/src/Pure/context.ML	Thu Oct 16 22:44:26 2008 +0200
@@ -356,7 +356,7 @@
   in thy' end;
 
 fun maximal_thys thys =
-  thys |> filter (fn thy => not (exists (fn thy' => proper_subthy (thy, thy')) thys));
+  thys |> filter_out (fn thy => exists (fn thy' => proper_subthy (thy, thy')) thys);
 
 fun begin_thy pp name imports =
   if name = draftN then error ("Illegal theory name: " ^ quote draftN)