adding a minimally refined equality on sets for code generation

--- a/src/HOL/List.thy	Thu Feb 02 10:12:11 2012 +0100
+++ b/src/HOL/List.thy	Thu Feb 02 10:12:30 2012 +0100
@@ -5676,6 +5676,11 @@
 
 text {* Further operations on sets *}
 
+(* Minimal refinement of equality on sets *)
+lemma [code]:
+  "HOL.equal (set []) (List.coset []) = False"
+by (metis UNIV_coset UNIV_not_empty empty_set equal_eq)
+
 lemma setsum_code [code]:
   "setsum f (set xs) = listsum (map f (remdups xs))"
 by (simp add: listsum_distinct_conv_setsum_set)