More porting to new locales.
1 (* Title: HOL/Real/HahnBanach/ZornLemma.thy
2 Author: Gertrud Bauer, TU Munich
5 header {* Zorn's Lemma *}
12 Zorn's Lemmas states: if every linear ordered subset of an ordered
13 set @{text S} has an upper bound in @{text S}, then there exists a
14 maximal element in @{text S}. In our application, @{text S} is a
15 set of sets ordered by set inclusion. Since the union of a chain of
16 sets is an upper bound for all elements of the chain, the conditions
17 of Zorn's lemma can be modified: if @{text S} is non-empty, it
18 suffices to show that for every non-empty chain @{text c} in @{text
19 S} the union of @{text c} also lies in @{text S}.
23 assumes r: "\<And>c. c \<in> chain S \<Longrightarrow> \<exists>x. x \<in> c \<Longrightarrow> \<Union>c \<in> S"
25 shows "\<exists>y \<in> S. \<forall>z \<in> S. y \<subseteq> z \<longrightarrow> y = z"
26 proof (rule Zorn_Lemma2)
27 show "\<forall>c \<in> chain S. \<exists>y \<in> S. \<forall>z \<in> c. z \<subseteq> y"
29 fix c assume "c \<in> chain S"
30 show "\<exists>y \<in> S. \<forall>z \<in> c. z \<subseteq> y"
33 txt {* If @{text c} is an empty chain, then every element in
34 @{text S} is an upper bound of @{text c}. *}
37 with aS show ?thesis by fast
39 txt {* If @{text c} is non-empty, then @{text "\<Union>c"} is an upper
40 bound of @{text c}, lying in @{text S}. *}
43 assume "c \<noteq> {}"
46 show "\<forall>z \<in> c. z \<subseteq> \<Union>c" by fast
47 show "\<Union>c \<in> S"
49 from `c \<noteq> {}` show "\<exists>x. x \<in> c" by fast
50 show "c \<in> chain S" by fact