diff -r 9a017146675f -r a354605f03dc src/HOL/Nat_Numeral.thy
--- a/src/HOL/Nat_Numeral.thy	Mon May 17 10:58:58 2010 +0200
+++ b/src/HOL/Nat_Numeral.thy	Mon May 17 08:40:17 2010 -0700
@@ -120,9 +120,9 @@
   "a\<twosuperior> = 0 \<longleftrightarrow> a = 0"
   unfolding power2_eq_square by simp
 
-lemma power2_eq_1_iff [simp]:
+lemma power2_eq_1_iff:
   "a\<twosuperior> = 1 \<longleftrightarrow> a = 1 \<or> a = - 1"
-  unfolding power2_eq_square by simp
+  unfolding power2_eq_square by (rule square_eq_1_iff)
 
 end