src/HOL/Library/Zorn.thy
 author chaieb Mon Feb 09 17:21:46 2009 +0000 (2009-02-09) changeset 29847 af32126ee729 parent 28952 15a4b2cf8c34 child 30198 922f944f03b2 permissions -rw-r--r--
```     1 (*  Title       : HOL/Library/Zorn.thy
```
```     2     Author      : Jacques D. Fleuriot, Tobias Nipkow
```
```     3     Description : Zorn's Lemma (ported from Larry Paulson's Zorn.thy in ZF)
```
```     4                   The well-ordering theorem
```
```     5 *)
```
```     6
```
```     7 header {* Zorn's Lemma *}
```
```     8
```
```     9 theory Zorn
```
```    10 imports "~~/src/HOL/Order_Relation"
```
```    11 begin
```
```    12
```
```    13 (* Define globally? In Set.thy? *)
```
```    14 definition chain_subset :: "'a set set \<Rightarrow> bool" ("chain\<^bsub>\<subseteq>\<^esub>") where
```
```    15 "chain\<^bsub>\<subseteq>\<^esub> C \<equiv> \<forall>A\<in>C.\<forall>B\<in>C. A \<subseteq> B \<or> B \<subseteq> A"
```
```    16
```
```    17 text{*
```
```    18   The lemma and section numbers refer to an unpublished article
```
```    19   \cite{Abrial-Laffitte}.
```
```    20 *}
```
```    21
```
```    22 definition
```
```    23   chain     ::  "'a set set => 'a set set set" where
```
```    24   "chain S  = {F. F \<subseteq> S & chain\<^bsub>\<subseteq>\<^esub> F}"
```
```    25
```
```    26 definition
```
```    27   super     ::  "['a set set,'a set set] => 'a set set set" where
```
```    28   "super S c = {d. d \<in> chain S & c \<subset> d}"
```
```    29
```
```    30 definition
```
```    31   maxchain  ::  "'a set set => 'a set set set" where
```
```    32   "maxchain S = {c. c \<in> chain S & super S c = {}}"
```
```    33
```
```    34 definition
```
```    35   succ      ::  "['a set set,'a set set] => 'a set set" where
```
```    36   "succ S c =
```
```    37     (if c \<notin> chain S | c \<in> maxchain S
```
```    38     then c else SOME c'. c' \<in> super S c)"
```
```    39
```
```    40 inductive_set
```
```    41   TFin :: "'a set set => 'a set set set"
```
```    42   for S :: "'a set set"
```
```    43   where
```
```    44     succI:        "x \<in> TFin S ==> succ S x \<in> TFin S"
```
```    45   | Pow_UnionI:   "Y \<in> Pow(TFin S) ==> Union(Y) \<in> TFin S"
```
```    46
```
```    47
```
```    48 subsection{*Mathematical Preamble*}
```
```    49
```
```    50 lemma Union_lemma0:
```
```    51     "(\<forall>x \<in> C. x \<subseteq> A | B \<subseteq> x) ==> Union(C) \<subseteq> A | B \<subseteq> Union(C)"
```
```    52   by blast
```
```    53
```
```    54
```
```    55 text{*This is theorem @{text increasingD2} of ZF/Zorn.thy*}
```
```    56
```
```    57 lemma Abrial_axiom1: "x \<subseteq> succ S x"
```
```    58   apply (auto simp add: succ_def super_def maxchain_def)
```
```    59   apply (rule contrapos_np, assumption)
```
```    60   apply (rule_tac Q="\<lambda>S. xa \<in> S" in someI2, blast+)
```
```    61   done
```
```    62
```
```    63 lemmas TFin_UnionI = TFin.Pow_UnionI [OF PowI]
```
```    64
```
```    65 lemma TFin_induct:
```
```    66   assumes H: "n \<in> TFin S"
```
```    67   and I: "!!x. x \<in> TFin S ==> P x ==> P (succ S x)"
```
```    68     "!!Y. Y \<subseteq> TFin S ==> Ball Y P ==> P(Union Y)"
```
```    69   shows "P n" using H
```
```    70   apply (induct rule: TFin.induct [where P=P])
```
```    71    apply (blast intro: I)+
```
```    72   done
```
```    73
```
```    74 lemma succ_trans: "x \<subseteq> y ==> x \<subseteq> succ S y"
```
```    75   apply (erule subset_trans)
```
```    76   apply (rule Abrial_axiom1)
```
```    77   done
```
```    78
```
```    79 text{*Lemma 1 of section 3.1*}
```
```    80 lemma TFin_linear_lemma1:
```
```    81      "[| n \<in> TFin S;  m \<in> TFin S;
```
```    82          \<forall>x \<in> TFin S. x \<subseteq> m --> x = m | succ S x \<subseteq> m
```
```    83       |] ==> n \<subseteq> m | succ S m \<subseteq> n"
```
```    84   apply (erule TFin_induct)
```
```    85    apply (erule_tac [2] Union_lemma0)
```
```    86   apply (blast del: subsetI intro: succ_trans)
```
```    87   done
```
```    88
```
```    89 text{* Lemma 2 of section 3.2 *}
```
```    90 lemma TFin_linear_lemma2:
```
```    91      "m \<in> TFin S ==> \<forall>n \<in> TFin S. n \<subseteq> m --> n=m | succ S n \<subseteq> m"
```
```    92   apply (erule TFin_induct)
```
```    93    apply (rule impI [THEN ballI])
```
```    94    txt{*case split using @{text TFin_linear_lemma1}*}
```
```    95    apply (rule_tac n1 = n and m1 = x in TFin_linear_lemma1 [THEN disjE],
```
```    96      assumption+)
```
```    97     apply (drule_tac x = n in bspec, assumption)
```
```    98     apply (blast del: subsetI intro: succ_trans, blast)
```
```    99   txt{*second induction step*}
```
```   100   apply (rule impI [THEN ballI])
```
```   101   apply (rule Union_lemma0 [THEN disjE])
```
```   102     apply (rule_tac [3] disjI2)
```
```   103     prefer 2 apply blast
```
```   104    apply (rule ballI)
```
```   105    apply (rule_tac n1 = n and m1 = x in TFin_linear_lemma1 [THEN disjE],
```
```   106      assumption+, auto)
```
```   107   apply (blast intro!: Abrial_axiom1 [THEN subsetD])
```
```   108   done
```
```   109
```
```   110 text{*Re-ordering the premises of Lemma 2*}
```
```   111 lemma TFin_subsetD:
```
```   112      "[| n \<subseteq> m;  m \<in> TFin S;  n \<in> TFin S |] ==> n=m | succ S n \<subseteq> m"
```
```   113   by (rule TFin_linear_lemma2 [rule_format])
```
```   114
```
```   115 text{*Consequences from section 3.3 -- Property 3.2, the ordering is total*}
```
```   116 lemma TFin_subset_linear: "[| m \<in> TFin S;  n \<in> TFin S|] ==> n \<subseteq> m | m \<subseteq> n"
```
```   117   apply (rule disjE)
```
```   118     apply (rule TFin_linear_lemma1 [OF _ _TFin_linear_lemma2])
```
```   119       apply (assumption+, erule disjI2)
```
```   120   apply (blast del: subsetI
```
```   121     intro: subsetI Abrial_axiom1 [THEN subset_trans])
```
```   122   done
```
```   123
```
```   124 text{*Lemma 3 of section 3.3*}
```
```   125 lemma eq_succ_upper: "[| n \<in> TFin S;  m \<in> TFin S;  m = succ S m |] ==> n \<subseteq> m"
```
```   126   apply (erule TFin_induct)
```
```   127    apply (drule TFin_subsetD)
```
```   128      apply (assumption+, force, blast)
```
```   129   done
```
```   130
```
```   131 text{*Property 3.3 of section 3.3*}
```
```   132 lemma equal_succ_Union: "m \<in> TFin S ==> (m = succ S m) = (m = Union(TFin S))"
```
```   133   apply (rule iffI)
```
```   134    apply (rule Union_upper [THEN equalityI])
```
```   135     apply assumption
```
```   136    apply (rule eq_succ_upper [THEN Union_least], assumption+)
```
```   137   apply (erule ssubst)
```
```   138   apply (rule Abrial_axiom1 [THEN equalityI])
```
```   139   apply (blast del: subsetI intro: subsetI TFin_UnionI TFin.succI)
```
```   140   done
```
```   141
```
```   142 subsection{*Hausdorff's Theorem: Every Set Contains a Maximal Chain.*}
```
```   143
```
```   144 text{*NB: We assume the partial ordering is @{text "\<subseteq>"},
```
```   145  the subset relation!*}
```
```   146
```
```   147 lemma empty_set_mem_chain: "({} :: 'a set set) \<in> chain S"
```
```   148 by (unfold chain_def chain_subset_def) auto
```
```   149
```
```   150 lemma super_subset_chain: "super S c \<subseteq> chain S"
```
```   151   by (unfold super_def) blast
```
```   152
```
```   153 lemma maxchain_subset_chain: "maxchain S \<subseteq> chain S"
```
```   154   by (unfold maxchain_def) blast
```
```   155
```
```   156 lemma mem_super_Ex: "c \<in> chain S - maxchain S ==> EX d. d \<in> super S c"
```
```   157   by (unfold super_def maxchain_def) auto
```
```   158
```
```   159 lemma select_super:
```
```   160      "c \<in> chain S - maxchain S ==> (\<some>c'. c': super S c): super S c"
```
```   161   apply (erule mem_super_Ex [THEN exE])
```
```   162   apply (rule someI2 [where Q="%X. X : super S c"], auto)
```
```   163   done
```
```   164
```
```   165 lemma select_not_equals:
```
```   166      "c \<in> chain S - maxchain S ==> (\<some>c'. c': super S c) \<noteq> c"
```
```   167   apply (rule notI)
```
```   168   apply (drule select_super)
```
```   169   apply (simp add: super_def less_le)
```
```   170   done
```
```   171
```
```   172 lemma succI3: "c \<in> chain S - maxchain S ==> succ S c = (\<some>c'. c': super S c)"
```
```   173   by (unfold succ_def) (blast intro!: if_not_P)
```
```   174
```
```   175 lemma succ_not_equals: "c \<in> chain S - maxchain S ==> succ S c \<noteq> c"
```
```   176   apply (frule succI3)
```
```   177   apply (simp (no_asm_simp))
```
```   178   apply (rule select_not_equals, assumption)
```
```   179   done
```
```   180
```
```   181 lemma TFin_chain_lemma4: "c \<in> TFin S ==> (c :: 'a set set): chain S"
```
```   182   apply (erule TFin_induct)
```
```   183    apply (simp add: succ_def select_super [THEN super_subset_chain[THEN subsetD]])
```
```   184   apply (unfold chain_def chain_subset_def)
```
```   185   apply (rule CollectI, safe)
```
```   186    apply (drule bspec, assumption)
```
```   187    apply (rule_tac [2] m1 = Xa and n1 = X in TFin_subset_linear [THEN disjE],
```
```   188      best+)
```
```   189   done
```
```   190
```
```   191 theorem Hausdorff: "\<exists>c. (c :: 'a set set): maxchain S"
```
```   192   apply (rule_tac x = "Union (TFin S)" in exI)
```
```   193   apply (rule classical)
```
```   194   apply (subgoal_tac "succ S (Union (TFin S)) = Union (TFin S) ")
```
```   195    prefer 2
```
```   196    apply (blast intro!: TFin_UnionI equal_succ_Union [THEN iffD2, symmetric])
```
```   197   apply (cut_tac subset_refl [THEN TFin_UnionI, THEN TFin_chain_lemma4])
```
```   198   apply (drule DiffI [THEN succ_not_equals], blast+)
```
```   199   done
```
```   200
```
```   201
```
```   202 subsection{*Zorn's Lemma: If All Chains Have Upper Bounds Then
```
```   203                                There Is  a Maximal Element*}
```
```   204
```
```   205 lemma chain_extend:
```
```   206   "[| c \<in> chain S; z \<in> S; \<forall>x \<in> c. x \<subseteq> (z:: 'a set) |] ==> {z} Un c \<in> chain S"
```
```   207 by (unfold chain_def chain_subset_def) blast
```
```   208
```
```   209 lemma chain_Union_upper: "[| c \<in> chain S; x \<in> c |] ==> x \<subseteq> Union(c)"
```
```   210 by auto
```
```   211
```
```   212 lemma chain_ball_Union_upper: "c \<in> chain S ==> \<forall>x \<in> c. x \<subseteq> Union(c)"
```
```   213 by auto
```
```   214
```
```   215 lemma maxchain_Zorn:
```
```   216   "[| c \<in> maxchain S; u \<in> S; Union(c) \<subseteq> u |] ==> Union(c) = u"
```
```   217 apply (rule ccontr)
```
```   218 apply (simp add: maxchain_def)
```
```   219 apply (erule conjE)
```
```   220 apply (subgoal_tac "({u} Un c) \<in> super S c")
```
```   221  apply simp
```
```   222 apply (unfold super_def less_le)
```
```   223 apply (blast intro: chain_extend dest: chain_Union_upper)
```
```   224 done
```
```   225
```
```   226 theorem Zorn_Lemma:
```
```   227   "\<forall>c \<in> chain S. Union(c): S ==> \<exists>y \<in> S. \<forall>z \<in> S. y \<subseteq> z --> y = z"
```
```   228 apply (cut_tac Hausdorff maxchain_subset_chain)
```
```   229 apply (erule exE)
```
```   230 apply (drule subsetD, assumption)
```
```   231 apply (drule bspec, assumption)
```
```   232 apply (rule_tac x = "Union(c)" in bexI)
```
```   233  apply (rule ballI, rule impI)
```
```   234  apply (blast dest!: maxchain_Zorn, assumption)
```
```   235 done
```
```   236
```
```   237 subsection{*Alternative version of Zorn's Lemma*}
```
```   238
```
```   239 lemma Zorn_Lemma2:
```
```   240   "\<forall>c \<in> chain S. \<exists>y \<in> S. \<forall>x \<in> c. x \<subseteq> y
```
```   241     ==> \<exists>y \<in> S. \<forall>x \<in> S. (y :: 'a set) \<subseteq> x --> y = x"
```
```   242 apply (cut_tac Hausdorff maxchain_subset_chain)
```
```   243 apply (erule exE)
```
```   244 apply (drule subsetD, assumption)
```
```   245 apply (drule bspec, assumption, erule bexE)
```
```   246 apply (rule_tac x = y in bexI)
```
```   247  prefer 2 apply assumption
```
```   248 apply clarify
```
```   249 apply (rule ccontr)
```
```   250 apply (frule_tac z = x in chain_extend)
```
```   251   apply (assumption, blast)
```
```   252 apply (unfold maxchain_def super_def less_le)
```
```   253 apply (blast elim!: equalityCE)
```
```   254 done
```
```   255
```
```   256 text{*Various other lemmas*}
```
```   257
```
```   258 lemma chainD: "[| c \<in> chain S; x \<in> c; y \<in> c |] ==> x \<subseteq> y | y \<subseteq> x"
```
```   259 by (unfold chain_def chain_subset_def) blast
```
```   260
```
```   261 lemma chainD2: "!!(c :: 'a set set). c \<in> chain S ==> c \<subseteq> S"
```
```   262 by (unfold chain_def) blast
```
```   263
```
```   264
```
```   265 (* Define globally? In Relation.thy? *)
```
```   266 definition Chain :: "('a*'a)set \<Rightarrow> 'a set set" where
```
```   267 "Chain r \<equiv> {A. \<forall>a\<in>A.\<forall>b\<in>A. (a,b) : r \<or> (b,a) \<in> r}"
```
```   268
```
```   269 lemma mono_Chain: "r \<subseteq> s \<Longrightarrow> Chain r \<subseteq> Chain s"
```
```   270 unfolding Chain_def by blast
```
```   271
```
```   272 text{* Zorn's lemma for partial orders: *}
```
```   273
```
```   274 lemma Zorns_po_lemma:
```
```   275 assumes po: "Partial_order r" and u: "\<forall>C\<in>Chain r. \<exists>u\<in>Field r. \<forall>a\<in>C. (a,u):r"
```
```   276 shows "\<exists>m\<in>Field r. \<forall>a\<in>Field r. (m,a):r \<longrightarrow> a=m"
```
```   277 proof-
```
```   278   have "Preorder r" using po by(simp add:partial_order_on_def)
```
```   279 --{* Mirror r in the set of subsets below (wrt r) elements of A*}
```
```   280   let ?B = "%x. r^-1 `` {x}" let ?S = "?B ` Field r"
```
```   281   have "\<forall>C \<in> chain ?S. EX U:?S. ALL A:C. A\<subseteq>U"
```
```   282   proof (auto simp:chain_def chain_subset_def)
```
```   283     fix C assume 1: "C \<subseteq> ?S" and 2: "\<forall>A\<in>C.\<forall>B\<in>C. A\<subseteq>B | B\<subseteq>A"
```
```   284     let ?A = "{x\<in>Field r. \<exists>M\<in>C. M = ?B x}"
```
```   285     have "C = ?B ` ?A" using 1 by(auto simp: image_def)
```
```   286     have "?A\<in>Chain r"
```
```   287     proof (simp add:Chain_def, intro allI impI, elim conjE)
```
```   288       fix a b
```
```   289       assume "a \<in> Field r" "?B a \<in> C" "b \<in> Field r" "?B b \<in> C"
```
```   290       hence "?B a \<subseteq> ?B b \<or> ?B b \<subseteq> ?B a" using 2 by auto
```
```   291       thus "(a, b) \<in> r \<or> (b, a) \<in> r" using `Preorder r` `a:Field r` `b:Field r`
```
```   292 	by(simp add:subset_Image1_Image1_iff)
```
```   293     qed
```
```   294     then obtain u where uA: "u:Field r" "\<forall>a\<in>?A. (a,u) : r" using u by auto
```
```   295     have "\<forall>A\<in>C. A \<subseteq> r^-1 `` {u}" (is "?P u")
```
```   296     proof auto
```
```   297       fix a B assume aB: "B:C" "a:B"
```
```   298       with 1 obtain x where "x:Field r" "B = r^-1 `` {x}" by auto
```
```   299       thus "(a,u) : r" using uA aB `Preorder r`
```
```   300 	by (auto simp add: preorder_on_def refl_def) (metis transD)
```
```   301     qed
```
```   302     thus "EX u:Field r. ?P u" using `u:Field r` by blast
```
```   303   qed
```
```   304   from Zorn_Lemma2[OF this]
```
```   305   obtain m B where "m:Field r" "B = r^-1 `` {m}"
```
```   306     "\<forall>x\<in>Field r. B \<subseteq> r^-1 `` {x} \<longrightarrow> B = r^-1 `` {x}"
```
```   307     by auto
```
```   308   hence "\<forall>a\<in>Field r. (m, a) \<in> r \<longrightarrow> a = m" using po `Preorder r` `m:Field r`
```
```   309     by(auto simp:subset_Image1_Image1_iff Partial_order_eq_Image1_Image1_iff)
```
```   310   thus ?thesis using `m:Field r` by blast
```
```   311 qed
```
```   312
```
```   313 (* The initial segment of a relation appears generally useful.
```
```   314    Move to Relation.thy?
```
```   315    Definition correct/most general?
```
```   316    Naming?
```
```   317 *)
```
```   318 definition init_seg_of :: "(('a*'a)set * ('a*'a)set)set" where
```
```   319 "init_seg_of == {(r,s). r \<subseteq> s \<and> (\<forall>a b c. (a,b):s \<and> (b,c):r \<longrightarrow> (a,b):r)}"
```
```   320
```
```   321 abbreviation initialSegmentOf :: "('a*'a)set \<Rightarrow> ('a*'a)set \<Rightarrow> bool"
```
```   322              (infix "initial'_segment'_of" 55) where
```
```   323 "r initial_segment_of s == (r,s):init_seg_of"
```
```   324
```
```   325 lemma refl_init_seg_of[simp]: "r initial_segment_of r"
```
```   326 by(simp add:init_seg_of_def)
```
```   327
```
```   328 lemma trans_init_seg_of:
```
```   329   "r initial_segment_of s \<Longrightarrow> s initial_segment_of t \<Longrightarrow> r initial_segment_of t"
```
```   330 by(simp (no_asm_use) add: init_seg_of_def)
```
```   331   (metis Domain_iff UnCI Un_absorb2 subset_trans)
```
```   332
```
```   333 lemma antisym_init_seg_of:
```
```   334   "r initial_segment_of s \<Longrightarrow> s initial_segment_of r \<Longrightarrow> r=s"
```
```   335 by(auto simp:init_seg_of_def)
```
```   336
```
```   337 lemma Chain_init_seg_of_Union:
```
```   338   "R \<in> Chain init_seg_of \<Longrightarrow> r\<in>R \<Longrightarrow> r initial_segment_of \<Union>R"
```
```   339 by(auto simp add:init_seg_of_def Chain_def Ball_def) blast
```
```   340
```
```   341 lemma chain_subset_trans_Union:
```
```   342   "chain\<^bsub>\<subseteq>\<^esub> R \<Longrightarrow> \<forall>r\<in>R. trans r \<Longrightarrow> trans(\<Union>R)"
```
```   343 apply(auto simp add:chain_subset_def)
```
```   344 apply(simp (no_asm_use) add:trans_def)
```
```   345 apply (metis subsetD)
```
```   346 done
```
```   347
```
```   348 lemma chain_subset_antisym_Union:
```
```   349   "chain\<^bsub>\<subseteq>\<^esub> R \<Longrightarrow> \<forall>r\<in>R. antisym r \<Longrightarrow> antisym(\<Union>R)"
```
```   350 apply(auto simp add:chain_subset_def antisym_def)
```
```   351 apply (metis subsetD)
```
```   352 done
```
```   353
```
```   354 lemma chain_subset_Total_Union:
```
```   355 assumes "chain\<^bsub>\<subseteq>\<^esub> R" "\<forall>r\<in>R. Total r"
```
```   356 shows "Total (\<Union>R)"
```
```   357 proof (simp add: total_on_def Ball_def, auto del:disjCI)
```
```   358   fix r s a b assume A: "r:R" "s:R" "a:Field r" "b:Field s" "a\<noteq>b"
```
```   359   from `chain\<^bsub>\<subseteq>\<^esub> R` `r:R` `s:R` have "r\<subseteq>s \<or> s\<subseteq>r"
```
```   360     by(simp add:chain_subset_def)
```
```   361   thus "(\<exists>r\<in>R. (a,b) \<in> r) \<or> (\<exists>r\<in>R. (b,a) \<in> r)"
```
```   362   proof
```
```   363     assume "r\<subseteq>s" hence "(a,b):s \<or> (b,a):s" using assms(2) A
```
```   364       by(simp add:total_on_def)(metis mono_Field subsetD)
```
```   365     thus ?thesis using `s:R` by blast
```
```   366   next
```
```   367     assume "s\<subseteq>r" hence "(a,b):r \<or> (b,a):r" using assms(2) A
```
```   368       by(simp add:total_on_def)(metis mono_Field subsetD)
```
```   369     thus ?thesis using `r:R` by blast
```
```   370   qed
```
```   371 qed
```
```   372
```
```   373 lemma wf_Union_wf_init_segs:
```
```   374 assumes "R \<in> Chain init_seg_of" and "\<forall>r\<in>R. wf r" shows "wf(\<Union>R)"
```
```   375 proof(simp add:wf_iff_no_infinite_down_chain, rule ccontr, auto)
```
```   376   fix f assume 1: "\<forall>i. \<exists>r\<in>R. (f(Suc i), f i) \<in> r"
```
```   377   then obtain r where "r:R" and "(f(Suc 0), f 0) : r" by auto
```
```   378   { fix i have "(f(Suc i), f i) \<in> r"
```
```   379     proof(induct i)
```
```   380       case 0 show ?case by fact
```
```   381     next
```
```   382       case (Suc i)
```
```   383       moreover obtain s where "s\<in>R" and "(f(Suc(Suc i)), f(Suc i)) \<in> s"
```
```   384 	using 1 by auto
```
```   385       moreover hence "s initial_segment_of r \<or> r initial_segment_of s"
```
```   386 	using assms(1) `r:R` by(simp add: Chain_def)
```
```   387       ultimately show ?case by(simp add:init_seg_of_def) blast
```
```   388     qed
```
```   389   }
```
```   390   thus False using assms(2) `r:R`
```
```   391     by(simp add:wf_iff_no_infinite_down_chain) blast
```
```   392 qed
```
```   393
```
```   394 lemma initial_segment_of_Diff:
```
```   395   "p initial_segment_of q \<Longrightarrow> p - s initial_segment_of q - s"
```
```   396 unfolding init_seg_of_def by blast
```
```   397
```
```   398 lemma Chain_inits_DiffI:
```
```   399   "R \<in> Chain init_seg_of \<Longrightarrow> {r - s |r. r \<in> R} \<in> Chain init_seg_of"
```
```   400 unfolding Chain_def by (blast intro: initial_segment_of_Diff)
```
```   401
```
```   402 theorem well_ordering: "\<exists>r::('a*'a)set. Well_order r \<and> Field r = UNIV"
```
```   403 proof-
```
```   404 -- {*The initial segment relation on well-orders: *}
```
```   405   let ?WO = "{r::('a*'a)set. Well_order r}"
```
```   406   def I \<equiv> "init_seg_of \<inter> ?WO \<times> ?WO"
```
```   407   have I_init: "I \<subseteq> init_seg_of" by(auto simp:I_def)
```
```   408   hence subch: "!!R. R : Chain I \<Longrightarrow> chain\<^bsub>\<subseteq>\<^esub> R"
```
```   409     by(auto simp:init_seg_of_def chain_subset_def Chain_def)
```
```   410   have Chain_wo: "!!R r. R \<in> Chain I \<Longrightarrow> r \<in> R \<Longrightarrow> Well_order r"
```
```   411     by(simp add:Chain_def I_def) blast
```
```   412   have FI: "Field I = ?WO" by(auto simp add:I_def init_seg_of_def Field_def)
```
```   413   hence 0: "Partial_order I"
```
```   414     by(auto simp: partial_order_on_def preorder_on_def antisym_def antisym_init_seg_of refl_def trans_def I_def elim!: trans_init_seg_of)
```
```   415 -- {*I-chains have upper bounds in ?WO wrt I: their Union*}
```
```   416   { fix R assume "R \<in> Chain I"
```
```   417     hence Ris: "R \<in> Chain init_seg_of" using mono_Chain[OF I_init] by blast
```
```   418     have subch: "chain\<^bsub>\<subseteq>\<^esub> R" using `R : Chain I` I_init
```
```   419       by(auto simp:init_seg_of_def chain_subset_def Chain_def)
```
```   420     have "\<forall>r\<in>R. Refl r" "\<forall>r\<in>R. trans r" "\<forall>r\<in>R. antisym r" "\<forall>r\<in>R. Total r"
```
```   421          "\<forall>r\<in>R. wf(r-Id)"
```
```   422       using Chain_wo[OF `R \<in> Chain I`] by(simp_all add:order_on_defs)
```
```   423     have "Refl (\<Union>R)" using `\<forall>r\<in>R. Refl r` by(auto simp:refl_def)
```
```   424     moreover have "trans (\<Union>R)"
```
```   425       by(rule chain_subset_trans_Union[OF subch `\<forall>r\<in>R. trans r`])
```
```   426     moreover have "antisym(\<Union>R)"
```
```   427       by(rule chain_subset_antisym_Union[OF subch `\<forall>r\<in>R. antisym r`])
```
```   428     moreover have "Total (\<Union>R)"
```
```   429       by(rule chain_subset_Total_Union[OF subch `\<forall>r\<in>R. Total r`])
```
```   430     moreover have "wf((\<Union>R)-Id)"
```
```   431     proof-
```
```   432       have "(\<Union>R)-Id = \<Union>{r-Id|r. r \<in> R}" by blast
```
```   433       with `\<forall>r\<in>R. wf(r-Id)` wf_Union_wf_init_segs[OF Chain_inits_DiffI[OF Ris]]
```
```   434       show ?thesis by (simp (no_asm_simp)) blast
```
```   435     qed
```
```   436     ultimately have "Well_order (\<Union>R)" by(simp add:order_on_defs)
```
```   437     moreover have "\<forall>r \<in> R. r initial_segment_of \<Union>R" using Ris
```
```   438       by(simp add: Chain_init_seg_of_Union)
```
```   439     ultimately have "\<Union>R : ?WO \<and> (\<forall>r\<in>R. (r,\<Union>R) : I)"
```
```   440       using mono_Chain[OF I_init] `R \<in> Chain I`
```
```   441       by(simp (no_asm) add:I_def del:Field_Union)(metis Chain_wo subsetD)
```
```   442   }
```
```   443   hence 1: "\<forall>R \<in> Chain I. \<exists>u\<in>Field I. \<forall>r\<in>R. (r,u) : I" by (subst FI) blast
```
```   444 --{*Zorn's Lemma yields a maximal well-order m:*}
```
```   445   then obtain m::"('a*'a)set" where "Well_order m" and
```
```   446     max: "\<forall>r. Well_order r \<and> (m,r):I \<longrightarrow> r=m"
```
```   447     using Zorns_po_lemma[OF 0 1] by (auto simp:FI)
```
```   448 --{*Now show by contradiction that m covers the whole type:*}
```
```   449   { fix x::'a assume "x \<notin> Field m"
```
```   450 --{*We assume that x is not covered and extend m at the top with x*}
```
```   451     have "m \<noteq> {}"
```
```   452     proof
```
```   453       assume "m={}"
```
```   454       moreover have "Well_order {(x,x)}"
```
```   455 	by(simp add:order_on_defs refl_def trans_def antisym_def total_on_def Field_def Domain_def Range_def)
```
```   456       ultimately show False using max
```
```   457 	by (auto simp:I_def init_seg_of_def simp del:Field_insert)
```
```   458     qed
```
```   459     hence "Field m \<noteq> {}" by(auto simp:Field_def)
```
```   460     moreover have "wf(m-Id)" using `Well_order m`
```
```   461       by(simp add:well_order_on_def)
```
```   462 --{*The extension of m by x:*}
```
```   463     let ?s = "{(a,x)|a. a : Field m}" let ?m = "insert (x,x) m Un ?s"
```
```   464     have Fm: "Field ?m = insert x (Field m)"
```
```   465       apply(simp add:Field_insert Field_Un)
```
```   466       unfolding Field_def by auto
```
```   467     have "Refl m" "trans m" "antisym m" "Total m" "wf(m-Id)"
```
```   468       using `Well_order m` by(simp_all add:order_on_defs)
```
```   469 --{*We show that the extension is a well-order*}
```
```   470     have "Refl ?m" using `Refl m` Fm by(auto simp:refl_def)
```
```   471     moreover have "trans ?m" using `trans m` `x \<notin> Field m`
```
```   472       unfolding trans_def Field_def Domain_def Range_def by blast
```
```   473     moreover have "antisym ?m" using `antisym m` `x \<notin> Field m`
```
```   474       unfolding antisym_def Field_def Domain_def Range_def by blast
```
```   475     moreover have "Total ?m" using `Total m` Fm by(auto simp: total_on_def)
```
```   476     moreover have "wf(?m-Id)"
```
```   477     proof-
```
```   478       have "wf ?s" using `x \<notin> Field m`
```
```   479 	by(auto simp add:wf_eq_minimal Field_def Domain_def Range_def) metis
```
```   480       thus ?thesis using `wf(m-Id)` `x \<notin> Field m`
```
```   481 	wf_subset[OF `wf ?s` Diff_subset]
```
```   482 	by (fastsimp intro!: wf_Un simp add: Un_Diff Field_def)
```
```   483     qed
```
```   484     ultimately have "Well_order ?m" by(simp add:order_on_defs)
```
```   485 --{*We show that the extension is above m*}
```
```   486     moreover hence "(m,?m) : I" using `Well_order m` `x \<notin> Field m`
```
```   487       by(fastsimp simp:I_def init_seg_of_def Field_def Domain_def Range_def)
```
```   488     ultimately
```
```   489 --{*This contradicts maximality of m:*}
```
```   490     have False using max `x \<notin> Field m` unfolding Field_def by blast
```
```   491   }
```
```   492   hence "Field m = UNIV" by auto
```
```   493   moreover with `Well_order m` have "Well_order m" by simp
```
```   494   ultimately show ?thesis by blast
```
```   495 qed
```
```   496
```
```   497 corollary well_order_on: "\<exists>r::('a*'a)set. well_order_on A r"
```
```   498 proof -
```
```   499   obtain r::"('a*'a)set" where wo: "Well_order r" and univ: "Field r = UNIV"
```
```   500     using well_ordering[where 'a = "'a"] by blast
```
```   501   let ?r = "{(x,y). x:A & y:A & (x,y):r}"
```
```   502   have 1: "Field ?r = A" using wo univ
```
```   503     by(fastsimp simp: Field_def Domain_def Range_def order_on_defs refl_def)
```
```   504   have "Refl r" "trans r" "antisym r" "Total r" "wf(r-Id)"
```
```   505     using `Well_order r` by(simp_all add:order_on_defs)
```
```   506   have "Refl ?r" using `Refl r` by(auto simp:refl_def 1 univ)
```
```   507   moreover have "trans ?r" using `trans r`
```
```   508     unfolding trans_def by blast
```
```   509   moreover have "antisym ?r" using `antisym r`
```
```   510     unfolding antisym_def by blast
```
```   511   moreover have "Total ?r" using `Total r` by(simp add:total_on_def 1 univ)
```
```   512   moreover have "wf(?r - Id)" by(rule wf_subset[OF `wf(r-Id)`]) blast
```
```   513   ultimately have "Well_order ?r" by(simp add:order_on_defs)
```
```   514   with 1 show ?thesis by metis
```
```   515 qed
```
```   516
```
```   517 end
```