(* Title: HOL/Order_Relation.thy
Author: Tobias Nipkow
Author: Andrei Popescu, TU Muenchen
*)
section \<open>Orders as Relations\<close>
theory Order_Relation
imports Wfrec
begin
subsection \<open>Orders on a set\<close>
definition "preorder_on A r \<equiv> refl_on A r \<and> trans r"
definition "partial_order_on A r \<equiv> preorder_on A r \<and> antisym r"
definition "linear_order_on A r \<equiv> partial_order_on A r \<and> total_on A r"
definition "strict_linear_order_on A r \<equiv> trans r \<and> irrefl r \<and> total_on A r"
definition "well_order_on A r \<equiv> linear_order_on A r \<and> wf(r - Id)"
lemmas order_on_defs =
preorder_on_def partial_order_on_def linear_order_on_def
strict_linear_order_on_def well_order_on_def
lemma partial_order_onD:
assumes "partial_order_on A r" shows "refl_on A r" and "trans r" and "antisym r"
using assms unfolding partial_order_on_def preorder_on_def by auto
lemma preorder_on_empty[simp]: "preorder_on {} {}"
by (simp add: preorder_on_def trans_def)
lemma partial_order_on_empty[simp]: "partial_order_on {} {}"
by (simp add: partial_order_on_def)
lemma lnear_order_on_empty[simp]: "linear_order_on {} {}"
by (simp add: linear_order_on_def)
lemma well_order_on_empty[simp]: "well_order_on {} {}"
by (simp add: well_order_on_def)
lemma preorder_on_converse[simp]: "preorder_on A (r\<inverse>) = preorder_on A r"
by (simp add: preorder_on_def)
lemma partial_order_on_converse[simp]: "partial_order_on A (r\<inverse>) = partial_order_on A r"
by (simp add: partial_order_on_def)
lemma linear_order_on_converse[simp]: "linear_order_on A (r\<inverse>) = linear_order_on A r"
by (simp add: linear_order_on_def)
lemma strict_linear_order_on_diff_Id: "linear_order_on A r \<Longrightarrow> strict_linear_order_on A (r - Id)"
by (simp add: order_on_defs trans_diff_Id)
lemma linear_order_on_singleton [simp]: "linear_order_on {x} {(x, x)}"
by (simp add: order_on_defs)
lemma linear_order_on_acyclic:
assumes "linear_order_on A r"
shows "acyclic (r - Id)"
using strict_linear_order_on_diff_Id[OF assms]
by (auto simp add: acyclic_irrefl strict_linear_order_on_def)
lemma linear_order_on_well_order_on:
assumes "finite r"
shows "linear_order_on A r \<longleftrightarrow> well_order_on A r"
unfolding well_order_on_def
using assms finite_acyclic_wf[OF _ linear_order_on_acyclic, of r] by blast
subsection \<open>Orders on the field\<close>
abbreviation "Refl r \<equiv> refl_on (Field r) r"
abbreviation "Preorder r \<equiv> preorder_on (Field r) r"
abbreviation "Partial_order r \<equiv> partial_order_on (Field r) r"
abbreviation "Total r \<equiv> total_on (Field r) r"
abbreviation "Linear_order r \<equiv> linear_order_on (Field r) r"
abbreviation "Well_order r \<equiv> well_order_on (Field r) r"
lemma subset_Image_Image_iff:
"Preorder r \<Longrightarrow> A \<subseteq> Field r \<Longrightarrow> B \<subseteq> Field r \<Longrightarrow>
r `` A \<subseteq> r `` B \<longleftrightarrow> (\<forall>a\<in>A.\<exists>b\<in>B. (b, a) \<in> r)"
apply (simp add: preorder_on_def refl_on_def Image_def subset_eq)
apply (simp only: trans_def)
apply fast
done
lemma subset_Image1_Image1_iff:
"Preorder r \<Longrightarrow> a \<in> Field r \<Longrightarrow> b \<in> Field r \<Longrightarrow> r `` {a} \<subseteq> r `` {b} \<longleftrightarrow> (b, a) \<in> r"
by (simp add: subset_Image_Image_iff)
lemma Refl_antisym_eq_Image1_Image1_iff:
assumes "Refl r"
and as: "antisym r"
and abf: "a \<in> Field r" "b \<in> Field r"
shows "r `` {a} = r `` {b} \<longleftrightarrow> a = b"
(is "?lhs \<longleftrightarrow> ?rhs")
proof
assume ?lhs
then have *: "\<And>x. (a, x) \<in> r \<longleftrightarrow> (b, x) \<in> r"
by (simp add: set_eq_iff)
have "(a, a) \<in> r" "(b, b) \<in> r" using \<open>Refl r\<close> abf by (simp_all add: refl_on_def)
then have "(a, b) \<in> r" "(b, a) \<in> r" using *[of a] *[of b] by simp_all
then show ?rhs
using \<open>antisym r\<close>[unfolded antisym_def] by blast
next
assume ?rhs
then show ?lhs by fast
qed
lemma Partial_order_eq_Image1_Image1_iff:
"Partial_order r \<Longrightarrow> a \<in> Field r \<Longrightarrow> b \<in> Field r \<Longrightarrow> r `` {a} = r `` {b} \<longleftrightarrow> a = b"
by (auto simp: order_on_defs Refl_antisym_eq_Image1_Image1_iff)
lemma Total_Id_Field:
assumes "Total r"
and not_Id: "\<not> r \<subseteq> Id"
shows "Field r = Field (r - Id)"
using mono_Field[of "r - Id" r] Diff_subset[of r Id]
proof auto
fix a assume *: "a \<in> Field r"
from not_Id have "r \<noteq> {}" by fast
with not_Id obtain b and c where "b \<noteq> c \<and> (b,c) \<in> r" by auto
then have "b \<noteq> c \<and> {b, c} \<subseteq> Field r" by (auto simp: Field_def)
with * obtain d where "d \<in> Field r" "d \<noteq> a" by auto
with * \<open>Total r\<close> have "(a, d) \<in> r \<or> (d, a) \<in> r" by (simp add: total_on_def)
with \<open>d \<noteq> a\<close> show "a \<in> Field (r - Id)" unfolding Field_def by blast
qed
subsection\<open>Relations given by a predicate and the field\<close>
definition relation_of :: "('a \<Rightarrow> 'a \<Rightarrow> bool) \<Rightarrow> 'a set \<Rightarrow> ('a \<times> 'a) set"
where "relation_of P A \<equiv> { (a, b) \<in> A \<times> A. P a b }"
lemma Field_relation_of:
assumes "refl_on A (relation_of P A)" shows "Field (relation_of P A) = A"
using assms unfolding refl_on_def Field_def by auto
lemma partial_order_on_relation_ofI:
assumes refl: "\<And>a. a \<in> A \<Longrightarrow> P a a"
and trans: "\<And>a b c. \<lbrakk> a \<in> A; b \<in> A; c \<in> A \<rbrakk> \<Longrightarrow> P a b \<Longrightarrow> P b c \<Longrightarrow> P a c"
and antisym: "\<And>a b. \<lbrakk> a \<in> A; b \<in> A \<rbrakk> \<Longrightarrow> P a b \<Longrightarrow> P b a \<Longrightarrow> a = b"
shows "partial_order_on A (relation_of P A)"
proof -
from refl have "refl_on A (relation_of P A)"
unfolding refl_on_def relation_of_def by auto
moreover have "trans (relation_of P A)" and "antisym (relation_of P A)"
unfolding relation_of_def
by (auto intro: transI dest: trans, auto intro: antisymI dest: antisym)
ultimately show ?thesis
unfolding partial_order_on_def preorder_on_def by simp
qed
lemma Partial_order_relation_ofI:
assumes "partial_order_on A (relation_of P A)" shows "Partial_order (relation_of P A)"
using Field_relation_of assms partial_order_on_def preorder_on_def by fastforce
subsection \<open>Orders on a type\<close>
abbreviation "strict_linear_order \<equiv> strict_linear_order_on UNIV"
abbreviation "linear_order \<equiv> linear_order_on UNIV"
abbreviation "well_order \<equiv> well_order_on UNIV"
subsection \<open>Order-like relations\<close>
text \<open>
In this subsection, we develop basic concepts and results pertaining
to order-like relations, i.e., to reflexive and/or transitive and/or symmetric and/or
total relations. We also further define upper and lower bounds operators.
\<close>
subsubsection \<open>Auxiliaries\<close>
lemma refl_on_domain: "refl_on A r \<Longrightarrow> (a, b) \<in> r \<Longrightarrow> a \<in> A \<and> b \<in> A"
by (auto simp add: refl_on_def)
corollary well_order_on_domain: "well_order_on A r \<Longrightarrow> (a, b) \<in> r \<Longrightarrow> a \<in> A \<and> b \<in> A"
by (auto simp add: refl_on_domain order_on_defs)
lemma well_order_on_Field: "well_order_on A r \<Longrightarrow> A = Field r"
by (auto simp add: refl_on_def Field_def order_on_defs)
lemma well_order_on_Well_order: "well_order_on A r \<Longrightarrow> A = Field r \<and> Well_order r"
using well_order_on_Field [of A] by auto
lemma Total_subset_Id:
assumes "Total r"
and "r \<subseteq> Id"
shows "r = {} \<or> (\<exists>a. r = {(a, a)})"
proof -
have "\<exists>a. r = {(a, a)}" if "r \<noteq> {}"
proof -
from that obtain a b where ab: "(a, b) \<in> r" by fast
with \<open>r \<subseteq> Id\<close> have "a = b" by blast
with ab have aa: "(a, a) \<in> r" by simp
have "a = c \<and> a = d" if "(c, d) \<in> r" for c d
proof -
from that have "{a, c, d} \<subseteq> Field r"
using ab unfolding Field_def by blast
then have "((a, c) \<in> r \<or> (c, a) \<in> r \<or> a = c) \<and> ((a, d) \<in> r \<or> (d, a) \<in> r \<or> a = d)"
using \<open>Total r\<close> unfolding total_on_def by blast
with \<open>r \<subseteq> Id\<close> show ?thesis by blast
qed
then have "r \<subseteq> {(a, a)}" by auto
with aa show ?thesis by blast
qed
then show ?thesis by blast
qed
lemma Linear_order_in_diff_Id:
assumes "Linear_order r"
and "a \<in> Field r"
and "b \<in> Field r"
shows "(a, b) \<in> r \<longleftrightarrow> (b, a) \<notin> r - Id"
using assms unfolding order_on_defs total_on_def antisym_def Id_def refl_on_def by force
subsubsection \<open>The upper and lower bounds operators\<close>
text \<open>
Here we define upper (``above") and lower (``below") bounds operators. We
think of \<open>r\<close> as a \<^emph>\<open>non-strict\<close> relation. The suffix \<open>S\<close> at the names of
some operators indicates that the bounds are strict -- e.g., \<open>underS a\<close> is
the set of all strict lower bounds of \<open>a\<close> (w.r.t. \<open>r\<close>). Capitalization of
the first letter in the name reminds that the operator acts on sets, rather
than on individual elements.
\<close>
definition under :: "'a rel \<Rightarrow> 'a \<Rightarrow> 'a set"
where "under r a \<equiv> {b. (b, a) \<in> r}"
definition underS :: "'a rel \<Rightarrow> 'a \<Rightarrow> 'a set"
where "underS r a \<equiv> {b. b \<noteq> a \<and> (b, a) \<in> r}"
definition Under :: "'a rel \<Rightarrow> 'a set \<Rightarrow> 'a set"
where "Under r A \<equiv> {b \<in> Field r. \<forall>a \<in> A. (b, a) \<in> r}"
definition UnderS :: "'a rel \<Rightarrow> 'a set \<Rightarrow> 'a set"
where "UnderS r A \<equiv> {b \<in> Field r. \<forall>a \<in> A. b \<noteq> a \<and> (b, a) \<in> r}"
definition above :: "'a rel \<Rightarrow> 'a \<Rightarrow> 'a set"
where "above r a \<equiv> {b. (a, b) \<in> r}"
definition aboveS :: "'a rel \<Rightarrow> 'a \<Rightarrow> 'a set"
where "aboveS r a \<equiv> {b. b \<noteq> a \<and> (a, b) \<in> r}"
definition Above :: "'a rel \<Rightarrow> 'a set \<Rightarrow> 'a set"
where "Above r A \<equiv> {b \<in> Field r. \<forall>a \<in> A. (a, b) \<in> r}"
definition AboveS :: "'a rel \<Rightarrow> 'a set \<Rightarrow> 'a set"
where "AboveS r A \<equiv> {b \<in> Field r. \<forall>a \<in> A. b \<noteq> a \<and> (a, b) \<in> r}"
definition ofilter :: "'a rel \<Rightarrow> 'a set \<Rightarrow> bool"
where "ofilter r A \<equiv> A \<subseteq> Field r \<and> (\<forall>a \<in> A. under r a \<subseteq> A)"
text \<open>
Note: In the definitions of \<open>Above[S]\<close> and \<open>Under[S]\<close>, we bounded
comprehension by \<open>Field r\<close> in order to properly cover the case of \<open>A\<close> being
empty.
\<close>
lemma underS_subset_under: "underS r a \<subseteq> under r a"
by (auto simp add: underS_def under_def)
lemma underS_notIn: "a \<notin> underS r a"
by (simp add: underS_def)
lemma Refl_under_in: "Refl r \<Longrightarrow> a \<in> Field r \<Longrightarrow> a \<in> under r a"
by (simp add: refl_on_def under_def)
lemma AboveS_disjoint: "A \<inter> (AboveS r A) = {}"
by (auto simp add: AboveS_def)
lemma in_AboveS_underS: "a \<in> Field r \<Longrightarrow> a \<in> AboveS r (underS r a)"
by (auto simp add: AboveS_def underS_def)
lemma Refl_under_underS: "Refl r \<Longrightarrow> a \<in> Field r \<Longrightarrow> under r a = underS r a \<union> {a}"
unfolding under_def underS_def
using refl_on_def[of _ r] by fastforce
lemma underS_empty: "a \<notin> Field r \<Longrightarrow> underS r a = {}"
by (auto simp: Field_def underS_def)
lemma under_Field: "under r a \<subseteq> Field r"
by (auto simp: under_def Field_def)
lemma underS_Field: "underS r a \<subseteq> Field r"
by (auto simp: underS_def Field_def)
lemma underS_Field2: "a \<in> Field r \<Longrightarrow> underS r a \<subset> Field r"
using underS_notIn underS_Field by fast
lemma underS_Field3: "Field r \<noteq> {} \<Longrightarrow> underS r a \<subset> Field r"
by (cases "a \<in> Field r") (auto simp: underS_Field2 underS_empty)
lemma AboveS_Field: "AboveS r A \<subseteq> Field r"
by (auto simp: AboveS_def Field_def)
lemma under_incr:
assumes "trans r"
and "(a, b) \<in> r"
shows "under r a \<subseteq> under r b"
unfolding under_def
proof auto
fix x assume "(x, a) \<in> r"
with assms trans_def[of r] show "(x, b) \<in> r" by blast
qed
lemma underS_incr:
assumes "trans r"
and "antisym r"
and ab: "(a, b) \<in> r"
shows "underS r a \<subseteq> underS r b"
unfolding underS_def
proof auto
assume *: "b \<noteq> a" and **: "(b, a) \<in> r"
with \<open>antisym r\<close> antisym_def[of r] ab show False
by blast
next
fix x assume "x \<noteq> a" "(x, a) \<in> r"
with ab \<open>trans r\<close> trans_def[of r] show "(x, b) \<in> r"
by blast
qed
lemma underS_incl_iff:
assumes LO: "Linear_order r"
and INa: "a \<in> Field r"
and INb: "b \<in> Field r"
shows "underS r a \<subseteq> underS r b \<longleftrightarrow> (a, b) \<in> r"
(is "?lhs \<longleftrightarrow> ?rhs")
proof
assume ?rhs
with \<open>Linear_order r\<close> show ?lhs
by (simp add: order_on_defs underS_incr)
next
assume *: ?lhs
have "(a, b) \<in> r" if "a = b"
using assms that by (simp add: order_on_defs refl_on_def)
moreover have False if "a \<noteq> b" "(b, a) \<in> r"
proof -
from that have "b \<in> underS r a" unfolding underS_def by blast
with * have "b \<in> underS r b" by blast
then show ?thesis by (simp add: underS_notIn)
qed
ultimately show "(a,b) \<in> r"
using assms order_on_defs[of "Field r" r] total_on_def[of "Field r" r] by blast
qed
lemma finite_Linear_order_induct[consumes 3, case_names step]:
assumes "Linear_order r"
and "x \<in> Field r"
and "finite r"
and step: "\<And>x. x \<in> Field r \<Longrightarrow> (\<And>y. y \<in> aboveS r x \<Longrightarrow> P y) \<Longrightarrow> P x"
shows "P x"
using assms(2)
proof (induct rule: wf_induct[of "r\<inverse> - Id"])
case 1
from assms(1,3) show "wf (r\<inverse> - Id)"
using linear_order_on_well_order_on linear_order_on_converse
unfolding well_order_on_def by blast
next
case prems: (2 x)
show ?case
by (rule step) (use prems in \<open>auto simp: aboveS_def intro: FieldI2\<close>)
qed
subsection \<open>Variations on Well-Founded Relations\<close>
text \<open>
This subsection contains some variations of the results from \<^theory>\<open>HOL.Wellfounded\<close>:
\<^item> means for slightly more direct definitions by well-founded recursion;
\<^item> variations of well-founded induction;
\<^item> means for proving a linear order to be a well-order.
\<close>
subsubsection \<open>Characterizations of well-foundedness\<close>
text \<open>
A transitive relation is well-founded iff it is ``locally'' well-founded,
i.e., iff its restriction to the lower bounds of of any element is
well-founded.
\<close>
lemma trans_wf_iff:
assumes "trans r"
shows "wf r \<longleftrightarrow> (\<forall>a. wf (r \<inter> (r\<inverse>``{a} \<times> r\<inverse>``{a})))"
proof -
define R where "R a = r \<inter> (r\<inverse>``{a} \<times> r\<inverse>``{a})" for a
have "wf (R a)" if "wf r" for a
using that R_def wf_subset[of r "R a"] by auto
moreover
have "wf r" if *: "\<forall>a. wf(R a)"
unfolding wf_def
proof clarify
fix phi a
assume **: "\<forall>a. (\<forall>b. (b, a) \<in> r \<longrightarrow> phi b) \<longrightarrow> phi a"
define chi where "chi b \<longleftrightarrow> (b, a) \<in> r \<longrightarrow> phi b" for b
with * have "wf (R a)" by auto
then have "(\<forall>b. (\<forall>c. (c, b) \<in> R a \<longrightarrow> chi c) \<longrightarrow> chi b) \<longrightarrow> (\<forall>b. chi b)"
unfolding wf_def by blast
also have "\<forall>b. (\<forall>c. (c, b) \<in> R a \<longrightarrow> chi c) \<longrightarrow> chi b"
proof (auto simp add: chi_def R_def)
fix b
assume "(b, a) \<in> r" and "\<forall>c. (c, b) \<in> r \<and> (c, a) \<in> r \<longrightarrow> phi c"
then have "\<forall>c. (c, b) \<in> r \<longrightarrow> phi c"
using assms trans_def[of r] by blast
with ** show "phi b" by blast
qed
finally have "\<forall>b. chi b" .
with ** chi_def show "phi a" by blast
qed
ultimately show ?thesis unfolding R_def by blast
qed
text\<open>A transitive relation is well-founded if all initial segments are finite.\<close>
corollary wf_finite_segments:
assumes "irrefl r" and "trans r" and "\<And>x. finite {y. (y, x) \<in> r}"
shows "wf (r)"
proof (clarsimp simp: trans_wf_iff wf_iff_acyclic_if_finite converse_def assms)
fix a
have "trans (r \<inter> ({x. (x, a) \<in> r} \<times> {x. (x, a) \<in> r}))"
using assms unfolding trans_def Field_def by blast
then show "acyclic (r \<inter> {x. (x, a) \<in> r} \<times> {x. (x, a) \<in> r})"
using assms acyclic_def assms irrefl_def by fastforce
qed
text \<open>The next lemma is a variation of \<open>wf_eq_minimal\<close> from Wellfounded,
allowing one to assume the set included in the field.\<close>
lemma wf_eq_minimal2: "wf r \<longleftrightarrow> (\<forall>A. A \<subseteq> Field r \<and> A \<noteq> {} \<longrightarrow> (\<exists>a \<in> A. \<forall>a' \<in> A. (a', a) \<notin> r))"
proof-
let ?phi = "\<lambda>A. A \<noteq> {} \<longrightarrow> (\<exists>a \<in> A. \<forall>a' \<in> A. (a',a) \<notin> r)"
have "wf r \<longleftrightarrow> (\<forall>A. ?phi A)"
apply (auto simp: ex_in_conv [THEN sym])
apply (erule wfE_min)
apply assumption
apply blast
apply (rule wfI_min)
apply fast
done
also have "(\<forall>A. ?phi A) \<longleftrightarrow> (\<forall>B \<subseteq> Field r. ?phi B)"
proof
assume "\<forall>A. ?phi A"
then show "\<forall>B \<subseteq> Field r. ?phi B" by simp
next
assume *: "\<forall>B \<subseteq> Field r. ?phi B"
show "\<forall>A. ?phi A"
proof clarify
fix A :: "'a set"
assume **: "A \<noteq> {}"
define B where "B = A \<inter> Field r"
show "\<exists>a \<in> A. \<forall>a' \<in> A. (a', a) \<notin> r"
proof (cases "B = {}")
case True
with ** obtain a where a: "a \<in> A" "a \<notin> Field r"
unfolding B_def by blast
with a have "\<forall>a' \<in> A. (a',a) \<notin> r"
unfolding Field_def by blast
with a show ?thesis by blast
next
case False
have "B \<subseteq> Field r" unfolding B_def by blast
with False * obtain a where a: "a \<in> B" "\<forall>a' \<in> B. (a', a) \<notin> r"
by blast
have "(a', a) \<notin> r" if "a' \<in> A" for a'
proof
assume a'a: "(a', a) \<in> r"
with that have "a' \<in> B" unfolding B_def Field_def by blast
with a a'a show False by blast
qed
with a show ?thesis unfolding B_def by blast
qed
qed
qed
finally show ?thesis by blast
qed
subsubsection \<open>Characterizations of well-foundedness\<close>
text \<open>
The next lemma and its corollary enable one to prove that a linear order is
a well-order in a way which is more standard than via well-foundedness of
the strict version of the relation.
\<close>
lemma Linear_order_wf_diff_Id:
assumes "Linear_order r"
shows "wf (r - Id) \<longleftrightarrow> (\<forall>A \<subseteq> Field r. A \<noteq> {} \<longrightarrow> (\<exists>a \<in> A. \<forall>a' \<in> A. (a, a') \<in> r))"
proof (cases "r \<subseteq> Id")
case True
then have *: "r - Id = {}" by blast
have "wf (r - Id)" by (simp add: *)
moreover have "\<exists>a \<in> A. \<forall>a' \<in> A. (a, a') \<in> r"
if *: "A \<subseteq> Field r" and **: "A \<noteq> {}" for A
proof -
from \<open>Linear_order r\<close> True
obtain a where a: "r = {} \<or> r = {(a, a)}"
unfolding order_on_defs using Total_subset_Id [of r] by blast
with * ** have "A = {a} \<and> r = {(a, a)}"
unfolding Field_def by blast
with a show ?thesis by blast
qed
ultimately show ?thesis by blast
next
case False
with \<open>Linear_order r\<close> have Field: "Field r = Field (r - Id)"
unfolding order_on_defs using Total_Id_Field [of r] by blast
show ?thesis
proof
assume *: "wf (r - Id)"
show "\<forall>A \<subseteq> Field r. A \<noteq> {} \<longrightarrow> (\<exists>a \<in> A. \<forall>a' \<in> A. (a, a') \<in> r)"
proof clarify
fix A
assume **: "A \<subseteq> Field r" and ***: "A \<noteq> {}"
then have "\<exists>a \<in> A. \<forall>a' \<in> A. (a',a) \<notin> r - Id"
using Field * unfolding wf_eq_minimal2 by simp
moreover have "\<forall>a \<in> A. \<forall>a' \<in> A. (a, a') \<in> r \<longleftrightarrow> (a', a) \<notin> r - Id"
using Linear_order_in_diff_Id [OF \<open>Linear_order r\<close>] ** by blast
ultimately show "\<exists>a \<in> A. \<forall>a' \<in> A. (a, a') \<in> r" by blast
qed
next
assume *: "\<forall>A \<subseteq> Field r. A \<noteq> {} \<longrightarrow> (\<exists>a \<in> A. \<forall>a' \<in> A. (a, a') \<in> r)"
show "wf (r - Id)"
unfolding wf_eq_minimal2
proof clarify
fix A
assume **: "A \<subseteq> Field(r - Id)" and ***: "A \<noteq> {}"
then have "\<exists>a \<in> A. \<forall>a' \<in> A. (a,a') \<in> r"
using Field * by simp
moreover have "\<forall>a \<in> A. \<forall>a' \<in> A. (a, a') \<in> r \<longleftrightarrow> (a', a) \<notin> r - Id"
using Linear_order_in_diff_Id [OF \<open>Linear_order r\<close>] ** mono_Field[of "r - Id" r] by blast
ultimately show "\<exists>a \<in> A. \<forall>a' \<in> A. (a',a) \<notin> r - Id"
by blast
qed
qed
qed
corollary Linear_order_Well_order_iff:
"Linear_order r \<Longrightarrow>
Well_order r \<longleftrightarrow> (\<forall>A \<subseteq> Field r. A \<noteq> {} \<longrightarrow> (\<exists>a \<in> A. \<forall>a' \<in> A. (a, a') \<in> r))"
unfolding well_order_on_def using Linear_order_wf_diff_Id[of r] by blast
end