changeset 24730 | a87d8d31abc0 |
parent 24658 | 49adbdcc52e2 |
child 25287 | 094dab519ff5 |
--- a/src/HOL/Set.thy Wed Sep 26 20:27:57 2007 +0200 +++ b/src/HOL/Set.thy Wed Sep 26 20:27:58 2007 +0200 @@ -771,6 +771,15 @@ lemma subset_insert_iff: "(A \<subseteq> insert x B) = (if x:A then A - {x} \<subseteq> B else A \<subseteq> B)" by auto +lemma set_insert: + assumes "x \<in> A" + obtains B where "A = insert x B" and "x \<notin> B" +proof + from assms show "A = insert x (A - {x})" by blast +next + show "x \<notin> A - {x}" by blast +qed + subsubsection {* Singletons, using insert *}