added monad instance for `set`

--- a/src/HOL/Library/Monad_Syntax.thy	Sat Dec 24 15:53:08 2011 +0100
+++ b/src/HOL/Library/Monad_Syntax.thy	Sat Dec 24 15:53:08 2011 +0100
@@ -71,6 +71,7 @@
 
 setup {*
   Adhoc_Overloading.add_overloaded @{const_name bind}
+  #> Adhoc_Overloading.add_variant @{const_name bind} @{const_name Set.bind}
   #> Adhoc_Overloading.add_variant @{const_name bind} @{const_name Predicate.bind}
   #> Adhoc_Overloading.add_variant @{const_name bind} @{const_name Option.bind}
   #> Adhoc_Overloading.add_variant @{const_name bind} @{const_name More_List.bind}