qualified_name_of: observe empty case;

--- a/src/Pure/General/binding.ML	Mon Mar 30 17:14:44 2009 +0200
+++ b/src/Pure/General/binding.ML	Mon Mar 30 19:51:10 2009 +0200
@@ -84,8 +84,9 @@
       let val (qualifier, name) = split_last (Long_Name.explode s)
       in make_binding ([], map (rpair false) qualifier, name, Position.none) end;
 
-fun qualified_name_of (Binding {qualifier, name, ...}) =
-  Long_Name.implode (map #1 qualifier @ [name]);
+fun qualified_name_of (b as Binding {qualifier, name, ...}) =
+  if is_empty b then ""
+  else Long_Name.implode (map #1 qualifier @ [name]);
 
 
 (* system prefix *)