diff -r 9ccfd1d1e874 -r 8d98b7711e47 src/HOL/Library/Nested_Environment.thy
--- a/src/HOL/Library/Nested_Environment.thy	Wed Jan 04 17:04:11 2006 +0100
+++ b/src/HOL/Library/Nested_Environment.thy	Wed Jan 04 19:22:53 2006 +0100
@@ -193,7 +193,7 @@
   from prems have "lookup env (xs @ ys) \<noteq> None" by simp
   then have "lookup env xs \<noteq> None"
     by (rule contrapos_nn) (simp only: lookup_append_none)
-  then show ?thesis by (simp add: not_None_eq)
+  then show ?thesis by (simp)
 qed
 
 text {*