added more explicit warning

--- a/src/HOL/Library/Executable_Set.thy	Fri Sep 03 08:13:28 2010 +0200
+++ b/src/HOL/Library/Executable_Set.thy	Sat Sep 04 21:10:39 2010 +0200
@@ -9,6 +9,12 @@
 imports More_Set
 begin
 
+text {*
+  This is just an ad-hoc hack which will rarely give you what you want.
+  For the moment, whenever you need executable sets, consider using
+  type @{text fset} from theory @{text Fset}.
+*}
+
 declare mem_def [code del]
 declare Collect_def [code del]
 declare insert_code [code del]