added map_cong to recdef

--- a/src/HOL/List.ML	Thu Aug 17 16:23:50 2000 +0200
+++ b/src/HOL/List.ML	Thu Aug 17 18:30:48 2000 +0200
@@ -326,6 +326,8 @@
 by Auto_tac;
 bind_thm("map_cong", impI RSN (2,allI RSN (2, result() RS mp)));
 
+Prim.Add_recdef_congs [map_cong];
+
 Goal "(map f xs = []) = (xs = [])";
 by (case_tac "xs" 1);
 by Auto_tac;