src/HOL/Tools/Function/fundef_lib.ML

      (K (rewrite_goals_tac ac
          THEN rtac Drule.reflexive_thm 1))
  end
 
 (* instance for unions *)
-fun regroup_union_conv t = regroup_conv @{const_name Set.empty} @{const_name Un}
-  (map (fn t => t RS eq_reflection) (@{thms "Un_ac"} @
-                                     @{thms "Un_empty_right"} @
-                                     @{thms "Un_empty_left"})) t
+fun regroup_union_conv t = regroup_conv @{const_name Set.empty} @{const_name Set.union}
+  (map (fn t => t RS eq_reflection) (@{thms Un_ac} @
+                                     @{thms Un_empty_right} @
+                                     @{thms Un_empty_left})) t
 
 
 end