added Coset as constructor

--- a/src/HOL/Library/Executable_Set.thy	Tue Oct 06 15:59:12 2009 +0200
+++ b/src/HOL/Library/Executable_Set.thy	Tue Oct 06 18:27:00 2009 +0200
@@ -74,11 +74,12 @@
 types_code
   fset ("(_/ \<module>fset)")
 attach {*
-datatype 'a fset = Set of 'a list;
+datatype 'a fset = Set of 'a list | Coset of 'a list;
 *}
 
 consts_code
   Set ("\<module>Set")
+  Coset ("\<module>Coset")
 
 consts_code
   "empty"             ("{*Fset.empty*}")