diff -r 787a203a20b6 -r c530cb79ccbc src/HOL/Rings.thy
--- a/src/HOL/Rings.thy	Mon Aug 22 21:37:06 2022 +0200
+++ b/src/HOL/Rings.thy	Wed Aug 24 06:21:06 2022 +0000
@@ -382,7 +382,7 @@
 
 subclass semiring_1_cancel ..
 
-lemma of_bool_not_iff [simp]:
+lemma of_bool_not_iff:
   \<open>of_bool (\<not> P) = 1 - of_bool P\<close>
   by simp