diff -r 6c558efcc754 -r da548623916a src/HOL/Library/Nested_Environment.thy
--- a/src/HOL/Library/Nested_Environment.thy	Tue Dec 20 22:06:00 2005 +0100
+++ b/src/HOL/Library/Nested_Environment.thy	Wed Dec 21 12:02:57 2005 +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
+  then show ?thesis by (simp add: not_None_eq)
 qed
 
 text {*