simplification procedures nat_cancel enabled by default;

--- a/src/HOL/arith_data.ML	Fri Dec 05 08:01:03 1997 +0100
+++ b/src/HOL/arith_data.ML	Fri Dec 05 17:13:46 1997 +0100
@@ -215,3 +215,7 @@
 
 
 open ArithData;
+
+
+context Arith.thy;
+Addsimprocs nat_cancel;