--- a/Nominal/NewFv.thy	Wed Apr 28 07:20:57 2010 +0200
+++ b/Nominal/NewFv.thy	Wed Apr 28 07:27:28 2010 +0200
@@ -26,7 +26,7 @@
 
 ML {*
 fun is_atom thy ty =
-  Sign.of_sort thy (ty, @{sort at})
+  Sign.of_sort thy (ty, @{sort at_base})
 
 fun is_atom_set thy (Type ("fun", [t, @{typ bool}])) = is_atom thy t
   | is_atom_set _ _ = false;