formal/afp/Abstract-Rewriting/Abstract_Rewriting.thy

(*  Title:       Abstract Rewriting
    Author:      Christian Sternagel <christian.sternagel@uibk.ac.at>
                 Rene Thiemann       <rene.tiemann@uibk.ac.at>
    Maintainer:  Christian Sternagel and Rene Thiemann
    License:     LGPL
*)

(*
Copyright 2010 Christian Sternagel and René Thiemann

This file is part of IsaFoR/CeTA.

IsaFoR/CeTA is free software: you can redistribute it and/or modify it under the
terms of the GNU Lesser General Public License as published by the Free Software
Foundation, either version 3 of the License, or (at your option) any later
version.

IsaFoR/CeTA is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.  See the GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License along
with IsaFoR/CeTA. If not, see <http://www.gnu.org/licenses/>.
*)

section \<open>Abstract Rewrite Systems\<close>

theory Abstract_Rewriting
imports
  "HOL-Library.Infinite_Set"
  "Regular-Sets.Regexp_Method"
  Seq
begin

(*FIXME: move*)
lemma trancl_mono_set:
  "r \<subseteq> s \<Longrightarrow> r\<^sup>+ \<subseteq> s\<^sup>+"
  by (blast intro: trancl_mono)

lemma relpow_mono:
  fixes r :: "'a rel"
  assumes "r \<subseteq> r'" shows "r ^^ n \<subseteq> r' ^^ n"
  using assms by (induct n) auto

lemma refl_inv_image:
  "refl R \<Longrightarrow> refl (inv_image R f)"
  by (simp add: inv_image_def refl_on_def)

subsection \<open>Definitions\<close>

text \<open>Two elements are \emph{joinable} (and then have in the joinability relation)
w.r.t.\ @{term "A"}, iff they have a common reduct.\<close>
definition join :: "'a rel \<Rightarrow> 'a rel"  ("(_\<^sup>\<down>)" [1000] 999) where
  "A\<^sup>\<down> = A\<^sup>* O (A\<inverse>)\<^sup>*"

text \<open>Two elements are \emph{meetable} (and then have in the meetability relation)
w.r.t.\ @{term "A"}, iff they have a common ancestor.\<close>
definition meet :: "'a rel \<Rightarrow> 'a rel"  ("(_\<^sup>\<up>)" [1000] 999) where
  "A\<^sup>\<up> = (A\<inverse>)\<^sup>* O A\<^sup>*"

text \<open>The \emph{symmetric closure} of a relation allows steps in both directions.\<close>
abbreviation symcl :: "'a rel \<Rightarrow> 'a rel"  ("(_\<^sup>\<leftrightarrow>)" [1000] 999) where
  "A\<^sup>\<leftrightarrow> \<equiv> A \<union> A\<inverse>"

text \<open>A \emph{conversion} is a (possibly empty) sequence of steps in the symmetric closure.\<close>
definition conversion :: "'a rel \<Rightarrow> 'a rel"  ("(_\<^sup>\<leftrightarrow>\<^sup>*)" [1000] 999) where
  "A\<^sup>\<leftrightarrow>\<^sup>* = (A\<^sup>\<leftrightarrow>)\<^sup>*"

text \<open>The set of \emph{normal forms} of an ARS constitutes all the elements that do
not have any successors.\<close>
definition NF :: "'a rel \<Rightarrow> 'a set" where
  "NF A = {a. A `` {a} = {}}"

definition normalizability :: "'a rel \<Rightarrow> 'a rel"  ("(_\<^sup>!)" [1000] 999) where
  "A\<^sup>! = {(a, b). (a, b) \<in> A\<^sup>* \<and> b \<in> NF A}"

notation (ASCII)
  symcl  ("(_^<->)" [1000] 999) and
  conversion  ("(_^<->*)" [1000] 999) and
  normalizability  ("(_^!)" [1000] 999)

lemma symcl_converse:
  "(A\<^sup>\<leftrightarrow>)\<inverse> = A\<^sup>\<leftrightarrow>" by auto

lemma symcl_Un: "(A \<union> B)\<^sup>\<leftrightarrow> = A\<^sup>\<leftrightarrow> \<union> B\<^sup>\<leftrightarrow>" by auto

lemma no_step:
  assumes "A `` {a} = {}" shows "a \<in> NF A"
  using assms by (auto simp: NF_def)

lemma joinI:
  "(a, c) \<in> A\<^sup>* \<Longrightarrow> (b, c) \<in> A\<^sup>* \<Longrightarrow> (a, b) \<in> A\<^sup>\<down>"
  by (auto simp: join_def rtrancl_converse)

lemma joinI_left:
  "(a, b) \<in> A\<^sup>* \<Longrightarrow> (a, b) \<in> A\<^sup>\<down>"
  by (auto simp: join_def)

lemma joinI_right: "(b, a) \<in> A\<^sup>* \<Longrightarrow> (a, b) \<in> A\<^sup>\<down>"
  by (rule joinI) auto

lemma joinE:
  assumes "(a, b) \<in> A\<^sup>\<down>"
  obtains c where "(a, c) \<in> A\<^sup>*" and "(b, c) \<in> A\<^sup>*"
  using assms by (auto simp: join_def rtrancl_converse)

lemma joinD:
  "(a, b) \<in> A\<^sup>\<down> \<Longrightarrow> \<exists>c. (a, c) \<in> A\<^sup>* \<and> (b, c) \<in> A\<^sup>*"
  by (blast elim: joinE)

lemma meetI:
  "(a, b) \<in> A\<^sup>* \<Longrightarrow> (a, c) \<in> A\<^sup>* \<Longrightarrow> (b, c) \<in> A\<^sup>\<up>"
  by (auto simp: meet_def rtrancl_converse)

lemma meetE:
  assumes "(b, c) \<in> A\<^sup>\<up>"
  obtains a where "(a, b) \<in> A\<^sup>*" and "(a, c) \<in> A\<^sup>*"
  using assms by (auto simp: meet_def rtrancl_converse)

lemma meetD: "(b, c) \<in> A\<^sup>\<up> \<Longrightarrow> \<exists>a. (a, b) \<in> A\<^sup>* \<and> (a, c) \<in> A\<^sup>*"
  by (blast elim: meetE)

lemma conversionI: "(a, b) \<in> (A\<^sup>\<leftrightarrow>)\<^sup>* \<Longrightarrow> (a, b) \<in> A\<^sup>\<leftrightarrow>\<^sup>*"
  by (simp add: conversion_def)

lemma conversion_refl [simp]: "(a, a) \<in> A\<^sup>\<leftrightarrow>\<^sup>*"
  by (simp add: conversion_def)

lemma conversionI':
  assumes "(a, b) \<in> A\<^sup>*" shows "(a, b) \<in> A\<^sup>\<leftrightarrow>\<^sup>*"
using assms
proof (induct)
  case base then show ?case by simp
next
  case (step b c)
  then have "(b, c) \<in> A\<^sup>\<leftrightarrow>" by simp
  with \<open>(a, b) \<in> A\<^sup>\<leftrightarrow>\<^sup>*\<close> show ?case unfolding conversion_def by (rule rtrancl.intros)
qed

lemma rtrancl_comp_trancl_conv:
  "r\<^sup>* O r = r\<^sup>+" by regexp

lemma trancl_o_refl_is_trancl:
  "r\<^sup>+ O r\<^sup>= = r\<^sup>+" by regexp

lemma conversionE:
  "(a, b) \<in> A\<^sup>\<leftrightarrow>\<^sup>* \<Longrightarrow> ((a, b) \<in> (A\<^sup>\<leftrightarrow>)\<^sup>* \<Longrightarrow> P) \<Longrightarrow> P"
  by (simp add: conversion_def)

text \<open>Later declarations are tried first for `proof' and `rule,' then have the ``main''
introduction\,/\, elimination rules for constants should be declared last.\<close>
declare joinI_left [intro]
declare joinI_right [intro]
declare joinI [intro]
declare joinD [dest]
declare joinE [elim]

declare meetI [intro]
declare meetD [dest]
declare meetE [elim]

declare conversionI' [intro]
declare conversionI [intro]
declare conversionE [elim]

lemma conversion_trans:
  "trans (A\<^sup>\<leftrightarrow>\<^sup>*)"
  unfolding trans_def
proof (intro allI impI)
  fix a b c assume "(a, b) \<in> A\<^sup>\<leftrightarrow>\<^sup>*" and "(b, c) \<in> A\<^sup>\<leftrightarrow>\<^sup>*"
  then show "(a, c) \<in> A\<^sup>\<leftrightarrow>\<^sup>*" unfolding conversion_def
  proof (induct)
    case base then show ?case by simp
  next
    case (step b c')
    from \<open>(b, c') \<in> A\<^sup>\<leftrightarrow>\<close> and \<open>(c', c) \<in> (A\<^sup>\<leftrightarrow>)\<^sup>*\<close>
      have "(b, c) \<in> (A\<^sup>\<leftrightarrow>)\<^sup>*" by (rule converse_rtrancl_into_rtrancl)
    with step show ?case by simp
  qed
qed

lemma conversion_sym:
  "sym (A\<^sup>\<leftrightarrow>\<^sup>*)"
  unfolding sym_def
proof (intro allI impI)
  fix a b assume "(a, b) \<in> A\<^sup>\<leftrightarrow>\<^sup>*" then show "(b, a) \<in> A\<^sup>\<leftrightarrow>\<^sup>*" unfolding conversion_def
  proof (induct)
    case base then show ?case by simp
  next
    case (step b c)
    then have "(c, b) \<in> A\<^sup>\<leftrightarrow>" by blast
    from \<open>(c, b) \<in> A\<^sup>\<leftrightarrow>\<close> and \<open>(b, a) \<in> (A\<^sup>\<leftrightarrow>)\<^sup>*\<close>
      show ?case by (rule converse_rtrancl_into_rtrancl)
  qed
qed

lemma conversion_inv:
  "(x, y) \<in> R\<^sup>\<leftrightarrow>\<^sup>* \<longleftrightarrow> (y, x) \<in> R\<^sup>\<leftrightarrow>\<^sup>*"
  by (auto simp: conversion_def)
     (metis (full_types) rtrancl_converseD symcl_converse)+


lemma conversion_converse [simp]:
  "(A\<^sup>\<leftrightarrow>\<^sup>*)\<inverse> = A\<^sup>\<leftrightarrow>\<^sup>*"
  by (metis conversion_sym sym_conv_converse_eq)

lemma conversion_rtrancl [simp]:
  "(A\<^sup>\<leftrightarrow>\<^sup>*)\<^sup>* = A\<^sup>\<leftrightarrow>\<^sup>*"
  by (metis conversion_def rtrancl_idemp)

lemma rtrancl_join_join:
  assumes "(a, b) \<in> A\<^sup>*" and "(b, c) \<in> A\<^sup>\<down>" shows "(a, c) \<in> A\<^sup>\<down>"
proof -
  from \<open>(b, c) \<in> A\<^sup>\<down>\<close> obtain b' where "(b, b') \<in> A\<^sup>*" and "(b', c) \<in> (A\<inverse>)\<^sup>*"
    unfolding join_def by blast
  with \<open>(a, b) \<in> A\<^sup>*\<close> have "(a, b') \<in> A\<^sup>*" by simp
  with \<open>(b', c) \<in> (A\<inverse>)\<^sup>*\<close> show ?thesis unfolding join_def by blast
qed

lemma join_rtrancl_join:
  assumes "(a, b) \<in> A\<^sup>\<down>" and "(c, b) \<in> A\<^sup>*" shows "(a, c) \<in> A\<^sup>\<down>"
proof -
  from \<open>(c, b) \<in> A\<^sup>*\<close> have "(b, c) \<in> (A\<inverse>)\<^sup>*" unfolding rtrancl_converse by simp
  from \<open>(a, b) \<in> A\<^sup>\<down>\<close> obtain a' where "(a, a') \<in> A\<^sup>*" and "(a', b) \<in> (A\<inverse>)\<^sup>*"
    unfolding join_def by best
  with \<open>(b, c) \<in> (A\<inverse>)\<^sup>*\<close> have "(a', c) \<in> (A\<inverse>)\<^sup>*" by simp
  with \<open>(a, a') \<in> A\<^sup>*\<close> show ?thesis unfolding join_def by blast
qed

lemma NF_I: "(\<And>b. (a, b) \<notin> A) \<Longrightarrow> a \<in> NF A" by (auto intro: no_step)

lemma NF_E: "a \<in> NF A \<Longrightarrow> ((a, b) \<notin> A \<Longrightarrow> P) \<Longrightarrow> P" by (auto simp: NF_def)

declare NF_I [intro]
declare NF_E [elim]

lemma NF_no_step: "a \<in> NF A \<Longrightarrow> \<forall>b. (a, b) \<notin> A" by auto

lemma NF_anti_mono:
  assumes "A \<subseteq> B" shows "NF B \<subseteq> NF A"
  using assms by auto

lemma NF_iff_no_step: "a \<in> NF A = (\<forall>b. (a, b) \<notin> A)" by auto

lemma NF_no_trancl_step:
  assumes "a \<in> NF A" shows "\<forall>b. (a, b) \<notin> A\<^sup>+"
proof -
  from assms have "\<forall>b. (a, b) \<notin> A" by auto
  show ?thesis
  proof (intro allI notI)
    fix b assume "(a, b) \<in> A\<^sup>+"
    then show False by (induct) (auto simp: \<open>\<forall>b. (a, b) \<notin> A\<close>)
   qed
qed

lemma NF_Id_on_fst_image [simp]: "NF (Id_on (fst ` A)) = NF A" by force

lemma fst_image_NF_Id_on [simp]: "fst ` R = Q \<Longrightarrow> NF (Id_on Q) = NF R" by force

lemma NF_empty [simp]: "NF {} = UNIV" by auto

lemma normalizability_I: "(a, b) \<in> A\<^sup>* \<Longrightarrow> b \<in> NF A \<Longrightarrow> (a, b) \<in> A\<^sup>!"
by (simp add: normalizability_def)

lemma normalizability_I': "(a, b) \<in> A\<^sup>* \<Longrightarrow> (b, c) \<in> A\<^sup>! \<Longrightarrow> (a, c) \<in> A\<^sup>!"
by (auto simp add: normalizability_def)

lemma normalizability_E: "(a, b) \<in> A\<^sup>! \<Longrightarrow> ((a, b) \<in> A\<^sup>* \<Longrightarrow> b \<in> NF A \<Longrightarrow> P) \<Longrightarrow> P"
by (simp add: normalizability_def)

declare normalizability_I' [intro]
declare normalizability_I [intro]
declare normalizability_E [elim]


subsection \<open>Properties of ARSs\<close>

text \<open>The following properties on (elements of) ARSs are defined: completeness,
Church-Rosser property, semi-completeness, strong normalization, unique normal
forms, Weak Church-Rosser property, and weak normalization.\<close>

definition CR_on :: "'a rel \<Rightarrow> 'a set \<Rightarrow> bool" where
  "CR_on r A \<longleftrightarrow> (\<forall>a\<in>A. \<forall>b c. (a, b) \<in> r\<^sup>* \<and> (a, c) \<in> r\<^sup>* \<longrightarrow> (b, c) \<in> join r)"

abbreviation CR :: "'a rel \<Rightarrow> bool" where
  "CR r \<equiv> CR_on r UNIV"

definition SN_on :: "'a rel \<Rightarrow> 'a set \<Rightarrow> bool" where
  "SN_on r A \<longleftrightarrow> \<not> (\<exists>f. f 0 \<in> A \<and> chain r f)"

abbreviation SN :: "'a rel \<Rightarrow> bool" where
  "SN r \<equiv> SN_on r UNIV"

text \<open>Alternative definition of @{term "SN"}.\<close>
lemma SN_def: "SN r = (\<forall>x. SN_on r {x})"
  unfolding SN_on_def by blast

definition UNF_on :: "'a rel \<Rightarrow> 'a set \<Rightarrow> bool" where
  "UNF_on r A \<longleftrightarrow> (\<forall>a\<in>A. \<forall>b c. (a, b) \<in> r\<^sup>! \<and> (a, c) \<in> r\<^sup>! \<longrightarrow> b = c)"

abbreviation UNF :: "'a rel \<Rightarrow> bool" where "UNF r \<equiv> UNF_on r UNIV"

definition WCR_on :: "'a rel \<Rightarrow> 'a set \<Rightarrow> bool" where
  "WCR_on r A \<longleftrightarrow> (\<forall>a\<in>A. \<forall>b c. (a, b) \<in> r \<and> (a, c) \<in> r \<longrightarrow> (b, c) \<in> join r)"

abbreviation WCR :: "'a rel \<Rightarrow> bool" where "WCR r \<equiv> WCR_on r UNIV"

definition WN_on :: "'a rel \<Rightarrow> 'a set \<Rightarrow> bool" where
  "WN_on r A \<longleftrightarrow> (\<forall>a\<in>A. \<exists>b. (a, b) \<in> r\<^sup>!)"

abbreviation WN :: "'a rel \<Rightarrow> bool" where
  "WN r \<equiv> WN_on r UNIV"

lemmas CR_defs = CR_on_def
lemmas SN_defs = SN_on_def
lemmas UNF_defs = UNF_on_def
lemmas WCR_defs = WCR_on_def
lemmas WN_defs = WN_on_def

definition complete_on :: "'a rel \<Rightarrow> 'a set \<Rightarrow> bool" where
  "complete_on r A \<longleftrightarrow> SN_on r A \<and> CR_on r A"

abbreviation complete :: "'a rel \<Rightarrow> bool" where
  "complete r \<equiv> complete_on r UNIV"

definition semi_complete_on :: "'a rel \<Rightarrow> 'a set \<Rightarrow> bool" where
  "semi_complete_on r A \<longleftrightarrow>  WN_on r A \<and> CR_on r A"

abbreviation semi_complete :: "'a rel \<Rightarrow> bool" where
  "semi_complete r \<equiv> semi_complete_on r UNIV"

lemmas complete_defs = complete_on_def
lemmas semi_complete_defs = semi_complete_on_def

text \<open>Unique normal forms with respect to conversion.\<close>
definition UNC :: "'a rel \<Rightarrow> bool" where
  "UNC A \<longleftrightarrow> (\<forall>a b. a \<in> NF A \<and> b \<in> NF A \<and> (a, b) \<in> A\<^sup>\<leftrightarrow>\<^sup>* \<longrightarrow> a = b)"

lemma complete_onI:
  "SN_on r A \<Longrightarrow> CR_on r A \<Longrightarrow> complete_on r A"
  by (simp add: complete_defs)

lemma complete_onE:
  "complete_on r A \<Longrightarrow> (SN_on r A \<Longrightarrow> CR_on r A \<Longrightarrow> P) \<Longrightarrow> P"
  by (simp add: complete_defs)

lemma CR_onI:
  "(\<And>a b c. a \<in> A \<Longrightarrow> (a, b) \<in> r\<^sup>* \<Longrightarrow> (a, c) \<in> r\<^sup>* \<Longrightarrow> (b, c) \<in> join r) \<Longrightarrow> CR_on r A"
  by (simp add: CR_defs)

lemma CR_on_singletonI:
  "(\<And>b c. (a, b) \<in> r\<^sup>* \<Longrightarrow> (a, c) \<in> r\<^sup>* \<Longrightarrow> (b, c) \<in> join r) \<Longrightarrow> CR_on r {a}"
  by (simp add: CR_defs)

lemma CR_onE:
  "CR_on r A \<Longrightarrow> a \<in> A \<Longrightarrow> ((b, c) \<in> join r \<Longrightarrow> P) \<Longrightarrow> ((a, b) \<notin> r\<^sup>* \<Longrightarrow> P) \<Longrightarrow> ((a, c) \<notin> r\<^sup>* \<Longrightarrow> P) \<Longrightarrow> P"
  unfolding CR_defs by blast

lemma CR_onD:
  "CR_on r A \<Longrightarrow> a \<in> A \<Longrightarrow> (a, b) \<in> r\<^sup>* \<Longrightarrow> (a, c) \<in> r\<^sup>* \<Longrightarrow> (b, c) \<in> join r"
  by (blast elim: CR_onE)

lemma semi_complete_onI: "WN_on r A \<Longrightarrow> CR_on r A \<Longrightarrow> semi_complete_on r A"
  by (simp add: semi_complete_defs)

lemma semi_complete_onE:
  "semi_complete_on r A \<Longrightarrow> (WN_on r A \<Longrightarrow> CR_on r A \<Longrightarrow> P) \<Longrightarrow> P"
  by (simp add: semi_complete_defs)

declare semi_complete_onI [intro]
declare semi_complete_onE [elim]

declare complete_onI [intro]
declare complete_onE [elim]

declare CR_onI [intro]
declare CR_on_singletonI [intro]

declare CR_onD [dest]
declare CR_onE [elim]

lemma UNC_I:
  "(\<And>a b. a \<in> NF A \<Longrightarrow> b \<in> NF A \<Longrightarrow> (a, b) \<in> A\<^sup>\<leftrightarrow>\<^sup>* \<Longrightarrow> a = b) \<Longrightarrow> UNC A"
  by (simp add: UNC_def)

lemma UNC_E:
  "\<lbrakk>UNC A; a = b \<Longrightarrow> P; a \<notin> NF A \<Longrightarrow> P; b \<notin> NF A \<Longrightarrow> P; (a, b) \<notin> A\<^sup>\<leftrightarrow>\<^sup>* \<Longrightarrow> P\<rbrakk> \<Longrightarrow> P"
  unfolding UNC_def by blast

lemma UNF_onI: "(\<And>a b c. a \<in> A \<Longrightarrow> (a, b) \<in> r\<^sup>! \<Longrightarrow> (a, c) \<in> r\<^sup>! \<Longrightarrow> b = c) \<Longrightarrow> UNF_on r A"
  by (simp add: UNF_defs)

lemma UNF_onE:
  "UNF_on r A \<Longrightarrow> a \<in> A \<Longrightarrow> (b = c \<Longrightarrow> P) \<Longrightarrow> ((a, b) \<notin> r\<^sup>! \<Longrightarrow> P) \<Longrightarrow> ((a, c) \<notin> r\<^sup>! \<Longrightarrow> P) \<Longrightarrow> P"
  unfolding UNF_on_def by blast

lemma UNF_onD:
  "UNF_on r A \<Longrightarrow> a \<in> A \<Longrightarrow> (a, b) \<in> r\<^sup>! \<Longrightarrow> (a, c) \<in> r\<^sup>! \<Longrightarrow> b = c"
  by (blast elim: UNF_onE)

declare UNF_onI [intro]
declare UNF_onD [dest]
declare UNF_onE [elim]

lemma SN_onI:
  assumes "\<And>f. \<lbrakk>f 0 \<in> A; chain r f\<rbrakk> \<Longrightarrow> False"
  shows "SN_on r A"
  using assms unfolding SN_defs by blast

lemma SN_I: "(\<And>a. SN_on A {a}) \<Longrightarrow> SN A"
  unfolding SN_on_def by blast

lemma SN_on_trancl_imp_SN_on:
  assumes "SN_on (R\<^sup>+) T" shows "SN_on R T"
proof (rule ccontr)
  assume "\<not> SN_on R T"
  then obtain s where "s 0 \<in> T" and "chain R s" unfolding SN_defs by auto
  then have "chain (R\<^sup>+) s" by auto
  with \<open>s 0 \<in> T\<close> have "\<not> SN_on (R\<^sup>+) T" unfolding SN_defs by auto
  with assms show False by simp
qed

lemma SN_onE:
  assumes "SN_on r A"
    and "\<not> (\<exists>f. f 0 \<in> A \<and> chain r f) \<Longrightarrow> P"
  shows "P"
  using assms unfolding SN_defs by simp

lemma not_SN_onE:
  assumes "\<not> SN_on r A"
    and "\<And>f. \<lbrakk>f 0 \<in> A; chain r f\<rbrakk> \<Longrightarrow> P"
  shows "P"
  using assms unfolding SN_defs by blast

declare SN_onI [intro]
declare SN_onE [elim]
declare not_SN_onE [Pure.elim, elim]

lemma refl_not_SN: "(x, x) \<in> R \<Longrightarrow> \<not> SN R"
  unfolding SN_defs by force

lemma SN_on_irrefl:
  assumes "SN_on r A"
  shows "\<forall>a\<in>A. (a, a) \<notin> r"
proof (intro ballI notI)
  fix a assume "a \<in> A" and "(a, a) \<in> r"
  with assms show False unfolding SN_defs by auto
qed

lemma WCR_onI: "(\<And>a b c. a \<in> A \<Longrightarrow> (a, b) \<in> r \<Longrightarrow> (a, c) \<in> r \<Longrightarrow> (b, c) \<in> join r) \<Longrightarrow> WCR_on r A"
  by (simp add: WCR_defs)

lemma WCR_onE:
  "WCR_on r A \<Longrightarrow> a \<in> A \<Longrightarrow> ((b, c) \<in> join r \<Longrightarrow> P) \<Longrightarrow> ((a, b) \<notin> r \<Longrightarrow> P) \<Longrightarrow> ((a, c) \<notin> r \<Longrightarrow> P) \<Longrightarrow> P"
  unfolding WCR_on_def by blast

lemma SN_nat_bounded: "SN {(x, y :: nat). x < y \<and> y \<le> b}" (is "SN ?R")
proof 
  fix f
  assume "chain ?R f"
  then have steps: "\<And>i. (f i, f (Suc i)) \<in> ?R" ..
  {
    fix i
    have inc: "f 0 + i \<le> f i"
    proof (induct i)
      case 0 then show ?case by auto
    next
      case (Suc i)
      have "f 0 + Suc i \<le> f i + Suc 0" using Suc by simp
      also have "... \<le> f (Suc i)" using steps [of i]
        by auto
      finally show ?case by simp
    qed
  }
  from this [of "Suc b"] steps [of b]
  show False by simp
qed

lemma WCR_onD:
  "WCR_on r A \<Longrightarrow> a \<in> A \<Longrightarrow> (a, b) \<in> r \<Longrightarrow> (a, c) \<in> r \<Longrightarrow> (b, c) \<in> join r"
  by (blast elim: WCR_onE)

lemma WN_onI: "(\<And>a. a \<in> A \<Longrightarrow> \<exists>b. (a, b) \<in> r\<^sup>!) \<Longrightarrow> WN_on r A"
  by (auto simp: WN_defs)

lemma WN_onE: "WN_on r A \<Longrightarrow> a \<in> A \<Longrightarrow> (\<And>b. (a, b) \<in> r\<^sup>! \<Longrightarrow> P) \<Longrightarrow> P"
 unfolding WN_defs by blast

lemma WN_onD: "WN_on r A \<Longrightarrow> a \<in> A \<Longrightarrow> \<exists>b. (a, b) \<in> r\<^sup>!"
  by (blast elim: WN_onE)

declare WCR_onI [intro]
declare WCR_onD [dest]
declare WCR_onE [elim]

declare WN_onI [intro]
declare WN_onD [dest]
declare WN_onE [elim]

text \<open>Restricting a relation @{term r} to those elements that are strongly
normalizing with respect to a relation @{term s}.\<close>
definition restrict_SN :: "'a rel \<Rightarrow> 'a rel \<Rightarrow> 'a rel" where
  "restrict_SN r s = {(a, b) | a b. (a, b) \<in> r \<and> SN_on s {a}}"

lemma SN_restrict_SN_idemp [simp]: "SN (restrict_SN A A)"
  by (auto simp: restrict_SN_def SN_defs)

lemma SN_on_Image:
  assumes "SN_on r A"
  shows "SN_on r (r `` A)"
proof
  fix f
  assume "f 0 \<in> r `` A" and chain: "chain r f"
  then obtain a where "a \<in> A" and 1: "(a, f 0) \<in> r" by auto
  let ?g = "case_nat a f"
  from cons_chain [OF 1 chain] have "chain r ?g" .
  moreover have "?g 0 \<in> A" by (simp add: \<open>a \<in> A\<close>)
  ultimately have "\<not> SN_on r A" unfolding SN_defs by best
  with assms show False by simp
qed

lemma SN_on_subset2:
  assumes "A \<subseteq> B" and "SN_on r B"
  shows "SN_on r A"
  using assms unfolding SN_on_def by blast

lemma step_preserves_SN_on:
  assumes 1: "(a, b) \<in> r"
    and 2: "SN_on r {a}"
  shows "SN_on r {b}"
  using 1 and SN_on_Image [OF 2] and SN_on_subset2 [of "{b}" "r `` {a}"] by auto

lemma steps_preserve_SN_on: "(a, b) \<in> A\<^sup>* \<Longrightarrow> SN_on A {a} \<Longrightarrow> SN_on A {b}"
  by (induct rule: rtrancl.induct) (auto simp: step_preserves_SN_on)

(*FIXME: move*)
lemma relpow_seq:
  assumes "(x, y) \<in> r^^n"
  shows "\<exists>f. f 0 = x \<and> f n = y \<and> (\<forall>i<n. (f i, f (Suc i)) \<in> r)"
using assms
proof (induct n arbitrary: y)
  case 0 then show ?case by auto
next
  case (Suc n)
  then obtain z where "(x, z) \<in> r^^n" and "(z, y) \<in> r" by auto
  from Suc(1)[OF \<open>(x, z) \<in> r^^n\<close>]
    obtain f where "f 0 = x" and "f n = z" and seq: "\<forall>i<n. (f i, f (Suc i)) \<in> r" by auto
  let ?n = "Suc n"
  let ?f = "\<lambda>i. if i = ?n then y else f i"
  have "?f ?n = y" by simp
  from \<open>f 0 = x\<close> have "?f 0 = x" by simp
  from seq have seq': "\<forall>i<n. (?f i, ?f (Suc i)) \<in> r" by auto
  with \<open>f n = z\<close> and \<open>(z, y) \<in> r\<close> have "\<forall>i<?n. (?f i, ?f (Suc i)) \<in> r" by auto
  with \<open>?f 0 = x\<close> and \<open>?f ?n = y\<close> show ?case by best
qed

lemma rtrancl_imp_seq:
  assumes "(x, y) \<in> r\<^sup>*"
  shows "\<exists>f n. f 0 = x \<and> f n = y \<and> (\<forall>i<n. (f i, f (Suc i)) \<in> r)"
  using assms [unfolded rtrancl_power] and relpow_seq [of x y _ r] by blast

lemma SN_on_Image_rtrancl:
  assumes "SN_on r A"
  shows "SN_on r (r\<^sup>* `` A)"
proof
  fix f
  assume f0: "f 0 \<in> r\<^sup>* `` A" and chain: "chain r f"
  then obtain a where a: "a \<in> A" and "(a, f 0) \<in> r\<^sup>*" by auto
  then obtain n where "(a, f 0) \<in> r^^n" unfolding rtrancl_power by auto
  show False
  proof (cases n)
    case 0
    with \<open>(a, f 0) \<in> r^^n\<close> have "f 0 = a" by simp
    then have "f 0 \<in> A" by (simp add: a)
    with chain have "\<not> SN_on r A" by auto
    with assms show False by simp
  next
    case (Suc m)
    from relpow_seq [OF \<open>(a, f 0) \<in> r^^n\<close>]
      obtain g where g0: "g 0 = a" and "g n = f 0"
      and gseq: "\<forall>i<n. (g i, g (Suc i)) \<in> r" by auto
    let ?f = "\<lambda>i. if i < n then g i else f (i - n)"
    have "chain r ?f"
    proof
      fix i
      {
        assume "Suc i < n"
        then have "(?f i, ?f (Suc i)) \<in> r" by (simp add: gseq)
      }
      moreover
      {
        assume "Suc i > n"
        then have eq: "Suc (i - n) = Suc i - n" by arith
        from chain have "(f (i - n), f (Suc (i - n))) \<in> r" by simp
        then have "(f (i - n), f (Suc i - n)) \<in> r" by (simp add: eq)
        with \<open>Suc i > n\<close> have "(?f i, ?f (Suc i)) \<in> r" by simp
      }
      moreover
      {
        assume "Suc i = n"
        then have eq: "f (Suc i - n) = g n" by (simp add: \<open>g n = f 0\<close>)
        from \<open>Suc i = n\<close> have eq': "i = n - 1" by arith
        from gseq have "(g i, f (Suc i - n)) \<in> r" unfolding eq by (simp add: Suc eq')
        then have "(?f i, ?f (Suc i)) \<in> r" using \<open>Suc i = n\<close> by simp
      }
      ultimately show "(?f i, ?f (Suc i)) \<in> r" by simp
    qed
    moreover have "?f 0 \<in> A"
    proof (cases n)
      case 0
      with \<open>(a, f 0) \<in> r^^n\<close> have eq: "a = f 0" by simp
      from a show ?thesis by (simp add: eq 0)
    next
      case (Suc m)
      then show ?thesis by (simp add: a g0)
    qed
    ultimately have "\<not> SN_on r A" unfolding SN_defs by best
    with assms show False by simp
  qed
qed

(* FIXME: move somewhere else *)
declare subrelI [Pure.intro]

lemma restrict_SN_trancl_simp [simp]: "(restrict_SN A A)\<^sup>+ = restrict_SN (A\<^sup>+) A" (is "?lhs = ?rhs")
proof
  show "?lhs \<subseteq> ?rhs"
  proof
    fix a b assume "(a, b) \<in> ?lhs" then show "(a, b) \<in> ?rhs"
      unfolding restrict_SN_def by (induct rule: trancl.induct) auto
  qed
next
  show "?rhs \<subseteq> ?lhs"
  proof
    fix a b assume "(a, b) \<in> ?rhs"
    then have "(a, b) \<in> A\<^sup>+" and "SN_on A {a}" unfolding restrict_SN_def by auto
    then show "(a, b) \<in> ?lhs"
    proof (induct rule: trancl.induct)
      case (r_into_trancl x y) then show ?case unfolding restrict_SN_def by auto
    next
      case (trancl_into_trancl a b c)
      then have IH: "(a, b) \<in> ?lhs" by auto
      from trancl_into_trancl have "(a, b) \<in> A\<^sup>*" by auto
      from this and \<open>SN_on A {a}\<close> have "SN_on A {b}" by (rule steps_preserve_SN_on)
      with \<open>(b, c) \<in> A\<close> have "(b, c) \<in> ?lhs" unfolding restrict_SN_def by auto
      with IH show ?case by simp
    qed
  qed
qed

lemma SN_imp_WN:
  assumes "SN A" shows "WN A"
proof -
  from \<open>SN A\<close> have "wf (A\<inverse>)" by (simp add: SN_defs wf_iff_no_infinite_down_chain)
  show "WN A"
  proof
    fix a
    show "\<exists>b. (a, b) \<in> A\<^sup>!" unfolding normalizability_def NF_def Image_def
      by (rule wfE_min [OF \<open>wf (A\<inverse>)\<close>, of a "A\<^sup>* `` {a}", simplified])
         (auto intro: rtrancl_into_rtrancl)
  qed
qed

lemma UNC_imp_UNF:
 assumes "UNC r" shows "UNF r"
proof - {
  fix x y z assume "(x, y) \<in> r\<^sup>!" and "(x, z) \<in> r\<^sup>!"
  then have "(x, y) \<in> r\<^sup>*" and "(x, z) \<in> r\<^sup>*" and "y \<in> NF r" and "z \<in> NF r" by auto
  then have "(x, y) \<in> r\<^sup>\<leftrightarrow>\<^sup>*" and "(x, z) \<in> r\<^sup>\<leftrightarrow>\<^sup>*" by auto
  then have "(z, x) \<in> r\<^sup>\<leftrightarrow>\<^sup>*" using conversion_sym unfolding sym_def by best
  with \<open>(x, y) \<in> r\<^sup>\<leftrightarrow>\<^sup>*\<close> have "(z, y) \<in> r\<^sup>\<leftrightarrow>\<^sup>*" using conversion_trans unfolding trans_def by best
  from assms and this and \<open>z \<in> NF r\<close> and \<open>y \<in> NF r\<close> have "z = y" unfolding UNC_def by auto
} then show ?thesis by auto
qed

lemma join_NF_imp_eq:
 assumes "(x, y) \<in> r\<^sup>\<down>" and "x \<in> NF r" and "y \<in> NF r"
 shows "x = y"
proof -
  from \<open>(x, y) \<in> r\<^sup>\<down>\<close> obtain z where "(x, z)\<in>r\<^sup>*" and "(z, y)\<in>(r\<inverse>)\<^sup>*" unfolding join_def by auto
  then have "(y, z) \<in> r\<^sup>*" unfolding rtrancl_converse by simp
  from \<open>x \<in> NF r\<close> have "(x, z) \<notin> r\<^sup>+" using NF_no_trancl_step by best
  then have "x = z" using rtranclD [OF \<open>(x, z) \<in> r\<^sup>*\<close>] by auto
  from \<open>y \<in> NF r\<close> have "(y, z) \<notin> r\<^sup>+" using NF_no_trancl_step by best
  then have "y = z" using rtranclD [OF \<open>(y, z) \<in> r\<^sup>*\<close>] by auto
  with \<open>x = z\<close> show ?thesis by simp
qed

lemma rtrancl_Restr:
  assumes "(x, y) \<in> (Restr r A)\<^sup>*"
  shows "(x, y) \<in> r\<^sup>*"
  using assms by induct auto

lemma join_mono:
  assumes "r \<subseteq> s"
  shows "r\<^sup>\<down> \<subseteq> s\<^sup>\<down>"
  using rtrancl_mono [OF assms] by (auto simp: join_def rtrancl_converse)


lemma CR_iff_meet_subset_join: "CR r = (r\<^sup>\<up> \<subseteq> r\<^sup>\<down>)"
proof
 assume "CR r" show "r\<^sup>\<up> \<subseteq> r\<^sup>\<down>"
 proof (rule subrelI)
  fix x y assume "(x, y) \<in> r\<^sup>\<up>"
  then obtain z where "(z, x) \<in> r\<^sup>*" and "(z, y) \<in> r\<^sup>*" using meetD by best
  with \<open>CR r\<close> show "(x, y) \<in> r\<^sup>\<down>" by (auto simp: CR_defs)
 qed
next
 assume "r\<^sup>\<up> \<subseteq> r\<^sup>\<down>" {
  fix x y z assume "(x, y) \<in> r\<^sup>*" and "(x, z) \<in> r\<^sup>*"
  then have "(y, z) \<in> r\<^sup>\<up>" unfolding meet_def rtrancl_converse by auto
  with \<open>r\<^sup>\<up> \<subseteq> r\<^sup>\<down>\<close> have "(y, z) \<in> r\<^sup>\<down>" by auto
 } then show "CR r" by (auto simp: CR_defs)
qed

lemma CR_divergence_imp_join:
  assumes "CR r" and "(x, y) \<in> r\<^sup>*" and "(x, z) \<in> r\<^sup>*"
  shows "(y, z) \<in> r\<^sup>\<down>"
using assms by auto

lemma join_imp_conversion: "r\<^sup>\<down> \<subseteq> r\<^sup>\<leftrightarrow>\<^sup>*"
proof
  fix x z assume "(x, z) \<in> r\<^sup>\<down>"
  then obtain y where "(x, y) \<in> r\<^sup>*" and "(z, y) \<in> r\<^sup>*" by auto
  then have "(x, y) \<in> r\<^sup>\<leftrightarrow>\<^sup>*" and "(z, y) \<in> r\<^sup>\<leftrightarrow>\<^sup>*" by auto
  from \<open>(z, y) \<in> r\<^sup>\<leftrightarrow>\<^sup>*\<close> have "(y, z) \<in> r\<^sup>\<leftrightarrow>\<^sup>*" using conversion_sym unfolding sym_def by best
  with \<open>(x, y) \<in> r\<^sup>\<leftrightarrow>\<^sup>*\<close> show "(x, z) \<in> r\<^sup>\<leftrightarrow>\<^sup>*" using conversion_trans unfolding trans_def by best
qed

lemma meet_imp_conversion: "r\<^sup>\<up> \<subseteq> r\<^sup>\<leftrightarrow>\<^sup>*"
proof (rule subrelI)
  fix y z assume "(y, z) \<in> r\<^sup>\<up>"
  then obtain x where "(x, y) \<in> r\<^sup>*" and "(x, z) \<in> r\<^sup>*" by auto
  then have "(x, y) \<in> r\<^sup>\<leftrightarrow>\<^sup>*" and "(x, z) \<in> r\<^sup>\<leftrightarrow>\<^sup>*" by auto
  from \<open>(x, y) \<in> r\<^sup>\<leftrightarrow>\<^sup>*\<close> have "(y, x) \<in> r\<^sup>\<leftrightarrow>\<^sup>*" using conversion_sym unfolding sym_def by best
  with \<open>(x, z) \<in> r\<^sup>\<leftrightarrow>\<^sup>*\<close> show "(y, z) \<in> r\<^sup>\<leftrightarrow>\<^sup>*" using conversion_trans unfolding trans_def by best
qed

lemma CR_imp_UNF:
  assumes "CR r" shows "UNF r"
proof - {
fix x y z assume "(x, y) \<in> r\<^sup>!" and "(x, z) \<in> r\<^sup>!"
  then have "(x, y) \<in> r\<^sup>*" and "y \<in> NF r" and "(x, z) \<in> r\<^sup>*" and "z \<in> NF r"
    unfolding normalizability_def by auto
  from assms and \<open>(x, y) \<in> r\<^sup>*\<close> and \<open>(x, z) \<in> r\<^sup>*\<close> have "(y, z) \<in> r\<^sup>\<down>"
    by (rule CR_divergence_imp_join)
  from this and \<open>y \<in> NF r\<close> and \<open>z \<in> NF r\<close> have "y = z" by (rule join_NF_imp_eq)
} then show ?thesis by auto
qed

lemma CR_iff_conversion_imp_join: "CR r = (r\<^sup>\<leftrightarrow>\<^sup>* \<subseteq> r\<^sup>\<down>)"
proof (intro iffI subrelI)
  fix x y assume "CR r" and "(x, y) \<in> r\<^sup>\<leftrightarrow>\<^sup>*"
  then obtain n where "(x, y) \<in> (r\<^sup>\<leftrightarrow>)^^n" unfolding conversion_def rtrancl_is_UN_relpow by auto
  then show "(x, y) \<in> r\<^sup>\<down>"
  proof (induct n arbitrary: x)
    case 0
    assume "(x, y) \<in> r\<^sup>\<leftrightarrow> ^^ 0" then have "x = y" by simp
    show ?case unfolding \<open>x = y\<close> by auto
  next
    case (Suc n)
    from \<open>(x, y) \<in> r\<^sup>\<leftrightarrow> ^^ Suc n\<close> obtain  z where "(x, z) \<in> r\<^sup>\<leftrightarrow>" and "(z, y) \<in> r\<^sup>\<leftrightarrow> ^^ n"
      using relpow_Suc_D2 by best
    with Suc have "(z, y) \<in> r\<^sup>\<down>" by simp
    from \<open>(x, z) \<in> r\<^sup>\<leftrightarrow>\<close> show ?case
    proof
      assume "(x, z) \<in> r" with \<open>(z, y) \<in> r\<^sup>\<down>\<close> show ?thesis by (auto intro: rtrancl_join_join)
    next
      assume "(x, z) \<in> r\<inverse>"
      then have "(z, x) \<in> r\<^sup>*" by simp
      from \<open>(z, y) \<in> r\<^sup>\<down>\<close> obtain z' where "(z, z') \<in> r\<^sup>*" and "(y, z') \<in> r\<^sup>*" by auto
      from \<open>CR r\<close> and \<open>(z, x) \<in> r\<^sup>*\<close> and \<open>(z, z') \<in> r\<^sup>*\<close> have "(x, z') \<in> r\<^sup>\<down>"
        by (rule CR_divergence_imp_join)
      then obtain x' where "(x, x') \<in> r\<^sup>*" and "(z', x') \<in> r\<^sup>*" by auto
      with \<open>(y, z') \<in> r\<^sup>*\<close> show ?thesis by auto
    qed
  qed
next
  assume "r\<^sup>\<leftrightarrow>\<^sup>* \<subseteq> r\<^sup>\<down>" then show "CR r" unfolding CR_iff_meet_subset_join
    using meet_imp_conversion by auto
qed

lemma CR_imp_conversionIff_join:
  assumes "CR r" shows "r\<^sup>\<leftrightarrow>\<^sup>* = r\<^sup>\<down>"
proof
  show "r\<^sup>\<leftrightarrow>\<^sup>* \<subseteq> r\<^sup>\<down>" using CR_iff_conversion_imp_join assms by auto
next
  show "r\<^sup>\<down> \<subseteq> r\<^sup>\<leftrightarrow>\<^sup>*" by (rule join_imp_conversion)
qed

lemma sym_join: "sym (join r)" by (auto simp: sym_def)

lemma join_sym: "(s, t) \<in> A\<^sup>\<down> \<Longrightarrow> (t, s) \<in> A\<^sup>\<down>" by auto

lemma CR_join_left_I:
  assumes "CR r" and "(x, y) \<in> r\<^sup>*" and "(x, z) \<in> r\<^sup>\<down>" shows "(y, z) \<in> r\<^sup>\<down>"
proof -
  from \<open>(x, z) \<in> r\<^sup>\<down>\<close> obtain x' where "(x, x') \<in> r\<^sup>*" and "(z, x') \<in> r\<^sup>\<down>" by auto
  from \<open>CR r\<close> and \<open>(x, x') \<in> r\<^sup>*\<close> and \<open>(x, y) \<in> r\<^sup>*\<close> have "(x, y) \<in> r\<^sup>\<down>" by auto
  then have "(y, x) \<in> r\<^sup>\<down>" using join_sym by best
  from \<open>CR r\<close> have "r\<^sup>\<leftrightarrow>\<^sup>* = r\<^sup>\<down>" by (rule CR_imp_conversionIff_join)
  from \<open>(y, x) \<in> r\<^sup>\<down>\<close> and \<open>(x, z) \<in> r\<^sup>\<down>\<close> show ?thesis using conversion_trans
    unfolding trans_def \<open>r\<^sup>\<leftrightarrow>\<^sup>* = r\<^sup>\<down>\<close> [symmetric] by best
qed

lemma CR_join_right_I:
 assumes "CR r" and "(x, y) \<in> r\<^sup>\<down>" and "(y, z) \<in> r\<^sup>*" shows "(x, z) \<in> r\<^sup>\<down>"
proof -
  have "r\<^sup>\<leftrightarrow>\<^sup>* = r\<^sup>\<down>" by (rule CR_imp_conversionIff_join [OF \<open>CR r\<close>])
  from \<open>(y, z) \<in> r\<^sup>*\<close> have "(y, z) \<in> r\<^sup>\<leftrightarrow>\<^sup>*" by auto
  with \<open>(x, y) \<in> r\<^sup>\<down>\<close> show ?thesis unfolding \<open>r\<^sup>\<leftrightarrow>\<^sup>* = r\<^sup>\<down>\<close> [symmetric] using conversion_trans
    unfolding trans_def by fast
qed

lemma NF_not_suc:
  assumes "(x, y) \<in> r\<^sup>*" and "x \<in> NF r" shows "x = y"
proof -
  from \<open>x \<in> NF r\<close> have "\<forall>y. (x, y) \<notin> r" using NF_no_step by auto
  then have "x \<notin> Domain r" unfolding Domain_unfold by simp
  from \<open>(x, y) \<in> r\<^sup>*\<close> show ?thesis unfolding Not_Domain_rtrancl [OF \<open>x \<notin> Domain r\<close>] by simp
qed

lemma semi_complete_imp_conversionIff_same_NF:
  assumes "semi_complete r"
  shows "((x, y) \<in> r\<^sup>\<leftrightarrow>\<^sup>*) = (\<forall>u v. (x, u) \<in> r\<^sup>! \<and> (y, v) \<in> r\<^sup>! \<longrightarrow> u = v)"
proof -
  from assms have "WN r" and "CR r" unfolding semi_complete_defs by auto
  then have "r\<^sup>\<leftrightarrow>\<^sup>* = r\<^sup>\<down>" using CR_imp_conversionIff_join by auto
  show ?thesis
  proof
    assume "(x, y) \<in> r\<^sup>\<leftrightarrow>\<^sup>*"
    from \<open>(x, y) \<in> r\<^sup>\<leftrightarrow>\<^sup>*\<close> have "(x, y) \<in> r\<^sup>\<down>" unfolding \<open>r\<^sup>\<leftrightarrow>\<^sup>* = r\<^sup>\<down>\<close> .
    show "\<forall>u v. (x, u) \<in> r\<^sup>! \<and> (y, v) \<in> r\<^sup>! \<longrightarrow> u = v"
    proof (intro allI impI, elim conjE)
      fix u v assume "(x, u) \<in> r\<^sup>!" and "(y, v) \<in> r\<^sup>!"
      then have "(x, u) \<in> r\<^sup>*" and "(y, v) \<in> r\<^sup>*" and "u \<in> NF r" and "v \<in> NF r" by auto
      from \<open>CR r\<close> and \<open>(x, u) \<in> r\<^sup>*\<close> and \<open>(x, y) \<in> r\<^sup>\<down>\<close> have "(u, y) \<in> r\<^sup>\<down>"
        by (auto intro: CR_join_left_I)
      then have "(y, u) \<in> r\<^sup>\<down>" using join_sym by best
      with \<open>(x, y) \<in> r\<^sup>\<down>\<close> have "(x, u) \<in> r\<^sup>\<down>" unfolding \<open>r\<^sup>\<leftrightarrow>\<^sup>* = r\<^sup>\<down>\<close> [symmetric]
        using conversion_trans unfolding trans_def by best
      from \<open>CR r\<close> and \<open>(x, y) \<in> r\<^sup>\<down>\<close> and \<open>(y, v) \<in> r\<^sup>*\<close> have "(x, v) \<in> r\<^sup>\<down>"
        by (auto intro: CR_join_right_I)
      then have "(v, x) \<in> r\<^sup>\<down>" using join_sym unfolding sym_def by best
      with \<open>(x, u) \<in> r\<^sup>\<down>\<close> have "(v, u) \<in> r\<^sup>\<down>" unfolding \<open>r\<^sup>\<leftrightarrow>\<^sup>* = r\<^sup>\<down>\<close> [symmetric]
        using conversion_trans unfolding trans_def by best
      then obtain v' where "(v, v') \<in> r\<^sup>*" and "(u, v') \<in> r\<^sup>*" by auto
      from \<open>(u, v') \<in> r\<^sup>*\<close> and \<open>u \<in> NF r\<close> have "u = v'" by (rule NF_not_suc)
      from \<open>(v, v') \<in> r\<^sup>*\<close> and \<open>v \<in> NF r\<close> have "v = v'" by (rule NF_not_suc)
      then show "u = v" unfolding \<open>u = v'\<close> by simp
    qed
  next
    assume equal_NF:"\<forall>u v. (x, u) \<in> r\<^sup>! \<and> (y, v) \<in> r\<^sup>! \<longrightarrow> u = v"
    from \<open>WN r\<close> obtain u where "(x, u) \<in> r\<^sup>!" by auto
    from \<open>WN r\<close> obtain v where "(y, v) \<in> r\<^sup>!" by auto
    from \<open>(x, u) \<in> r\<^sup>!\<close> and \<open>(y, v) \<in> r\<^sup>!\<close> have "u = v" using equal_NF by simp
    from \<open>(x, u) \<in> r\<^sup>!\<close> and \<open>(y, v) \<in> r\<^sup>!\<close> have "(x, v) \<in> r\<^sup>*" and "(y, v) \<in> r\<^sup>*"
      unfolding \<open>u = v\<close> by auto
    then have "(x, v) \<in> r\<^sup>\<leftrightarrow>\<^sup>*" and "(y, v) \<in> r\<^sup>\<leftrightarrow>\<^sup>*" by auto
    from \<open>(y, v) \<in> r\<^sup>\<leftrightarrow>\<^sup>*\<close> have "(v, y) \<in> r\<^sup>\<leftrightarrow>\<^sup>*" using conversion_sym unfolding sym_def by best
    with \<open>(x, v) \<in> r\<^sup>\<leftrightarrow>\<^sup>*\<close> show "(x, y) \<in> r\<^sup>\<leftrightarrow>\<^sup>*" using conversion_trans unfolding trans_def by best
  qed
qed

lemma CR_imp_UNC:
  assumes "CR r" shows "UNC r"
proof - {
  fix x y assume "x \<in> NF r" and "y \<in> NF r" and "(x, y) \<in> r\<^sup>\<leftrightarrow>\<^sup>*"
  have "r\<^sup>\<leftrightarrow>\<^sup>* = r\<^sup>\<down>" by (rule CR_imp_conversionIff_join [OF assms])
  from \<open>(x, y) \<in> r\<^sup>\<leftrightarrow>\<^sup>*\<close> have "(x, y) \<in> r\<^sup>\<down>" unfolding \<open>r\<^sup>\<leftrightarrow>\<^sup>* = r\<^sup>\<down>\<close> by simp
  then obtain x' where "(x, x') \<in> r\<^sup>*" and "(y, x') \<in> r\<^sup>*" by best
  from \<open>(x, x') \<in> r\<^sup>*\<close> and \<open>x \<in> NF r\<close> have "x = x'" by (rule NF_not_suc)
  from \<open>(y, x') \<in> r\<^sup>*\<close> and \<open>y \<in> NF r\<close> have "y = x'" by (rule NF_not_suc)
  then have "x = y" unfolding \<open>x = x'\<close> by simp
} then show ?thesis by (auto simp: UNC_def)
qed

lemma WN_UNF_imp_CR:
  assumes "WN r" and "UNF r" shows "CR r"
proof - {
  fix x y z assume "(x, y) \<in> r\<^sup>*" and "(x, z) \<in> r\<^sup>*"
  from assms obtain y' where "(y, y') \<in> r\<^sup>!" unfolding WN_defs by best
  with \<open>(x, y) \<in> r\<^sup>*\<close> have "(x, y') \<in> r\<^sup>!" by auto
  from assms obtain z' where "(z, z') \<in> r\<^sup>!" unfolding WN_defs by best
  with \<open>(x, z) \<in> r\<^sup>*\<close> have "(x, z') \<in> r\<^sup>!" by auto
  with \<open>(x, y') \<in> r\<^sup>!\<close> have "y' = z'" using \<open>UNF r\<close> unfolding UNF_defs by auto
  from \<open>(y, y') \<in> r\<^sup>!\<close> and \<open>(z, z') \<in> r\<^sup>!\<close> have "(y, z) \<in> r\<^sup>\<down>" unfolding \<open>y' = z'\<close> by auto
} then show ?thesis by auto
qed

definition diamond :: "'a rel \<Rightarrow> bool" ("\<diamond>") where
  "\<diamond> r \<longleftrightarrow> (r\<inverse> O r) \<subseteq> (r O r\<inverse>)"

lemma diamond_I [intro]: "(r\<inverse> O r) \<subseteq> (r O r\<inverse>) \<Longrightarrow> \<diamond> r" unfolding diamond_def by simp

lemma diamond_E [elim]: "\<diamond> r \<Longrightarrow> ((r\<inverse> O r) \<subseteq> (r O r\<inverse>) \<Longrightarrow> P) \<Longrightarrow> P"
  unfolding diamond_def by simp

lemma diamond_imp_semi_confluence:
  assumes "\<diamond> r" shows "(r\<inverse> O r\<^sup>*) \<subseteq> r\<^sup>\<down>"
proof (rule subrelI)
  fix y z assume "(y, z) \<in>  r\<inverse> O r\<^sup>*"
  then obtain x where "(x, y) \<in> r" and "(x, z) \<in> r\<^sup>*" by best
  then obtain n where "(x, z) \<in> r^^n" using rtrancl_imp_UN_relpow by best
  with \<open>(x, y) \<in> r\<close> show "(y, z) \<in> r\<^sup>\<down>"
  proof (induct n arbitrary: x z y)
    case 0 then show ?case by auto
  next
    case (Suc n)
    from \<open>(x, z) \<in> r^^Suc n\<close> obtain x' where "(x, x') \<in> r" and "(x', z) \<in> r^^n"
      using relpow_Suc_D2 by best
    with \<open>(x, y) \<in> r\<close> have "(y, x') \<in> (r\<inverse> O r)" by auto
    with \<open>\<diamond> r\<close> have "(y, x') \<in> (r O r\<inverse>)" by auto
    then obtain y' where "(x', y') \<in> r" and "(y, y') \<in> r" by best
    with Suc and \<open>(x', z) \<in> r^^n\<close> have "(y', z) \<in> r\<^sup>\<down>" by auto
    with \<open>(y, y') \<in> r\<close> show ?case by (auto intro: rtrancl_join_join)
  qed
qed

lemma semi_confluence_imp_CR:
  assumes "(r\<inverse> O r\<^sup>*) \<subseteq> r\<^sup>\<down>" shows "CR r"
proof - {
  fix x y z assume "(x, y) \<in> r\<^sup>*" and "(x, z) \<in> r\<^sup>*"
  then obtain n where "(x, z) \<in> r^^n" using rtrancl_imp_UN_relpow by best
  with \<open>(x, y) \<in> r\<^sup>*\<close> have "(y, z) \<in> r\<^sup>\<down>"
  proof (induct n arbitrary: x y z)
    case 0 then show ?case by auto
  next
    case (Suc n)
    from \<open>(x, z) \<in> r^^Suc n\<close> obtain x' where "(x, x') \<in> r" and "(x', z) \<in> r^^n"
      using relpow_Suc_D2 by best
    from \<open>(x, x') \<in> r\<close> and \<open>(x, y) \<in> r\<^sup>*\<close> have "(x', y) \<in> (r\<inverse> O r\<^sup>* )" by auto
    with assms have "(x', y) \<in> r\<^sup>\<down>" by auto
    then obtain y' where "(x', y') \<in> r\<^sup>*" and "(y, y') \<in> r\<^sup>*" by best
    with Suc and \<open>(x', z) \<in> r^^n\<close> have "(y', z) \<in> r\<^sup>\<down>" by simp
    then obtain u where "(z, u) \<in> r\<^sup>*" and "(y', u) \<in> r\<^sup>*" by best
    from \<open>(y, y') \<in> r\<^sup>*\<close> and \<open>(y', u) \<in> r\<^sup>*\<close> have "(y, u) \<in> r\<^sup>*" by auto
    with \<open>(z, u) \<in> r\<^sup>*\<close> show ?case by best
  qed
} then show ?thesis by auto
qed
 
lemma diamond_imp_CR:
  assumes "\<diamond> r" shows "CR r"
  using assms by (rule diamond_imp_semi_confluence [THEN semi_confluence_imp_CR])

lemma diamond_imp_CR':
  assumes "\<diamond> s" and "r \<subseteq> s" and "s \<subseteq> r\<^sup>*" shows "CR r"
  unfolding CR_iff_meet_subset_join
proof -
  from \<open>\<diamond> s\<close> have "CR s" by (rule diamond_imp_CR)
  then have "s\<^sup>\<up> \<subseteq> s\<^sup>\<down>" unfolding CR_iff_meet_subset_join by simp
  from \<open>r \<subseteq> s\<close> have "r\<^sup>* \<subseteq> s\<^sup>*" by (rule rtrancl_mono)
  from \<open>s \<subseteq> r\<^sup>*\<close> have "s\<^sup>* \<subseteq> (r\<^sup>*)\<^sup>*" by (rule rtrancl_mono)
  then have "s\<^sup>* \<subseteq> r\<^sup>*" by simp
  with \<open>r\<^sup>* \<subseteq> s\<^sup>*\<close> have "r\<^sup>* = s\<^sup>*" by simp
  show "r\<^sup>\<up> \<subseteq> r\<^sup>\<down>" unfolding meet_def join_def rtrancl_converse \<open>r\<^sup>* = s\<^sup>*\<close>
    unfolding rtrancl_converse [symmetric] meet_def [symmetric]
      join_def [symmetric] by (rule \<open>s\<^sup>\<up> \<subseteq> s\<^sup>\<down>\<close>)
qed

lemma SN_imp_minimal:
  assumes "SN A"
  shows "\<forall>Q x. x \<in> Q \<longrightarrow> (\<exists>z\<in>Q. \<forall>y. (z, y) \<in> A \<longrightarrow> y \<notin> Q)"
proof (rule ccontr)
  assume "\<not> (\<forall>Q x. x \<in> Q \<longrightarrow> (\<exists>z\<in>Q. \<forall>y. (z, y) \<in> A \<longrightarrow> y \<notin> Q))"
  then obtain Q x where "x \<in> Q" and "\<forall>z\<in>Q. \<exists>y. (z, y) \<in> A \<and> y \<in> Q" by auto
  then have "\<forall>z. \<exists>y. z \<in> Q \<longrightarrow> (z, y) \<in> A \<and> y \<in> Q" by auto
  then have "\<exists>f. \<forall>x. x \<in> Q \<longrightarrow> (x, f x) \<in> A \<and> f x \<in> Q" by (rule choice)
  then obtain f where a:"\<forall>x. x \<in> Q \<longrightarrow> (x, f x) \<in> A \<and> f x \<in> Q" (is "\<forall>x. ?P x") by best
  let ?S = "\<lambda>i. (f ^^ i) x"
  have "?S 0 = x" by simp
  have "\<forall>i. (?S i, ?S (Suc i)) \<in> A \<and> ?S (Suc i) \<in> Q"
  proof
    fix i show "(?S i, ?S (Suc i)) \<in> A \<and> ?S (Suc i) \<in> Q"
      by (induct i) (auto simp: \<open>x \<in> Q\<close> a)
  qed
  with \<open>?S 0 = x\<close> have "\<exists>S. S 0 = x \<and> chain A S" by fast
  with assms show False by auto
qed

lemma SN_on_imp_on_minimal:
  assumes "SN_on r {x}"
  shows "\<forall>Q. x \<in> Q \<longrightarrow> (\<exists>z\<in>Q. \<forall>y. (z, y) \<in> r \<longrightarrow> y \<notin> Q)"
proof (rule ccontr)
  assume "\<not>(\<forall>Q. x \<in> Q \<longrightarrow> (\<exists>z\<in>Q. \<forall>y. (z, y) \<in> r \<longrightarrow> y \<notin> Q))"
  then obtain Q where "x \<in> Q" and "\<forall>z\<in>Q. \<exists>y. (z, y) \<in> r \<and> y \<in> Q" by auto
  then have "\<forall>z. \<exists>y. z \<in> Q \<longrightarrow> (z, y) \<in> r \<and> y \<in> Q" by auto
  then have "\<exists>f. \<forall>x. x \<in> Q \<longrightarrow> (x, f x) \<in> r \<and> f x \<in> Q" by (rule choice)
  then obtain f where a: "\<forall>x. x \<in> Q \<longrightarrow> (x, f x) \<in> r \<and> f x \<in> Q" (is "\<forall>x. ?P x") by best
  let ?S = "\<lambda>i. (f ^^ i) x"
  have "?S 0 = x" by simp
  have "\<forall>i. (?S i,?S(Suc i)) \<in> r \<and> ?S(Suc i) \<in> Q"
  proof
    fix i show "(?S i,?S(Suc i)) \<in> r \<and> ?S(Suc i) \<in> Q" by (induct i) (auto simp:\<open>x \<in> Q\<close> a)
  qed
  with \<open>?S 0 = x\<close> have "\<exists>S. S 0 = x \<and> chain r S" by fast
  with assms show False by auto
qed

lemma minimal_imp_wf:
  assumes "\<forall>Q x. x \<in> Q \<longrightarrow> (\<exists>z\<in>Q. \<forall>y. (z, y) \<in> r \<longrightarrow> y \<notin> Q)"
  shows "wf(r\<inverse>)"
proof (rule ccontr)
  assume "\<not> wf(r\<inverse>)"
  then have "\<exists>P. (\<forall>x. (\<forall>y. (x, y) \<in> r \<longrightarrow> P y) \<longrightarrow> P x) \<and> (\<exists>x. \<not> P x)" unfolding wf_def by simp
  then obtain P x where suc:"\<forall>x. (\<forall>y. (x, y) \<in> r \<longrightarrow> P y) \<longrightarrow> P x" and "\<not> P x" by auto
  let ?Q = "{x. \<not> P x}"
  from \<open>\<not> P x\<close> have "x \<in> ?Q" by simp
  from assms have "\<forall>x. x \<in> ?Q \<longrightarrow> (\<exists>z\<in>?Q. \<forall>y. (z, y) \<in> r \<longrightarrow> y \<notin> ?Q)" by (rule allE [where x = ?Q])
  with \<open>x \<in> ?Q\<close> obtain z where "z \<in> ?Q" and min:" \<forall>y. (z, y) \<in> r \<longrightarrow> y \<notin> ?Q" by best
  from \<open>z \<in> ?Q\<close> have "\<not> P z" by simp
  with suc obtain y where "(z, y) \<in> r" and "\<not> P y" by best
  then have "y \<in> ?Q" by simp
  with \<open>(z, y) \<in> r\<close> and min show False by simp
qed

lemmas SN_imp_wf = SN_imp_minimal [THEN minimal_imp_wf]

lemma wf_imp_SN:
  assumes "wf (A\<inverse>)" shows "SN A"
proof - {
  fix a
  let ?P = "\<lambda>a. \<not>(\<exists>S. S 0 = a \<and> chain A S)"
  from \<open>wf (A\<inverse>)\<close> have "?P a"
  proof induct
    case (less a)
    then have IH: "\<And>b. (a, b) \<in> A \<Longrightarrow> ?P b" by auto
    show "?P a"
    proof (rule ccontr)
      assume "\<not> ?P a"
      then obtain S where "S 0 = a" and "chain A S" by auto
      then have "(S 0, S 1) \<in> A" by auto
      with IH have "?P (S 1)" unfolding \<open>S 0 = a\<close> by auto
      with \<open>chain A S\<close> show False by auto
    qed
  qed
  then have "SN_on A {a}" unfolding SN_defs by auto
} then show ?thesis by fast
qed

lemma SN_nat_gt: "SN {(a, b :: nat) . a > b}"
proof -
  from wf_less have "wf ({(x, y) . (x :: nat) > y}\<inverse>)" unfolding converse_unfold by auto
  from wf_imp_SN [OF this] show ?thesis .
qed


lemma SN_iff_wf: "SN A = wf (A\<inverse>)" by (auto simp: SN_imp_wf wf_imp_SN)

lemma SN_imp_acyclic: "SN R \<Longrightarrow> acyclic R"
  using wf_acyclic [of "R\<inverse>", unfolded SN_iff_wf [symmetric]] by auto

lemma SN_induct:
  assumes sn: "SN r" and step: "\<And>a. (\<And>b. (a, b) \<in> r \<Longrightarrow> P b) \<Longrightarrow> P a"
  shows "P a"
using sn unfolding SN_iff_wf proof induct
  case (less a)
  with step show ?case by best
qed

(* The same as well-founded induction, but in the 'correct' direction. *)
lemmas SN_induct_rule = SN_induct [consumes 1, case_names IH, induct pred: SN]

lemma SN_on_induct [consumes 2, case_names IH, induct pred: SN_on]:
  assumes SN: "SN_on R A"
    and "s \<in> A"
    and imp: "\<And>t. (\<And>u. (t, u) \<in> R \<Longrightarrow> P u) \<Longrightarrow> P t"
  shows "P s"
proof -
  let ?R = "restrict_SN R R"
  let ?P = "\<lambda>t. SN_on R {t} \<longrightarrow> P t"
  have "SN_on R {s} \<longrightarrow> P s"
  proof (rule SN_induct [OF SN_restrict_SN_idemp [of R], of ?P])
    fix a
    assume ind: "\<And>b. (a, b) \<in> ?R \<Longrightarrow> SN_on R {b} \<longrightarrow> P b"
    show "SN_on R {a} \<longrightarrow> P a"
    proof
      assume SN: "SN_on R {a}"
      show "P a"
      proof (rule imp)
        fix b
        assume "(a, b) \<in> R"
        with SN step_preserves_SN_on [OF this SN]
        show "P b" using ind [of b] unfolding restrict_SN_def by auto
      qed
    qed
  qed
  with SN show "P s" using \<open>s \<in> A\<close> unfolding SN_on_def by blast
qed

(* link SN_on to acc / accp *)
lemma accp_imp_SN_on:
  assumes "\<And>x. x \<in> A \<Longrightarrow> Wellfounded.accp g x"
  shows "SN_on {(y, z). g z y} A"
proof - {
  fix x assume "x \<in> A"
  from assms [OF this]
  have "SN_on {(y, z). g z y} {x}"
  proof (induct rule: accp.induct)
    case (accI x)
    show ?case
    proof
      fix f
      assume x: "f 0 \<in> {x}" and steps: "\<forall> i. (f i, f (Suc i)) \<in> {a. (\<lambda>(y, z). g z y) a}"
      then have "g (f 1) x" by auto
      from accI(2)[OF this] steps x show False unfolding SN_on_def by auto
    qed
  qed
  }
  then show ?thesis unfolding SN_on_def by blast
qed

lemma SN_on_imp_accp:
  assumes "SN_on {(y, z). g z y} A"
  shows "\<forall>x\<in>A. Wellfounded.accp g x"
proof
  fix x assume "x \<in> A"
  with assms show "Wellfounded.accp g x"
  proof (induct rule: SN_on_induct)
    case (IH x)
    show ?case
    proof
      fix y
      assume "g y x"
      with IH show "Wellfounded.accp g y" by simp
    qed
  qed
qed

lemma SN_on_conv_accp:
  "SN_on {(y, z). g z y} {x} = Wellfounded.accp g x"
  using SN_on_imp_accp [of g "{x}"]
        accp_imp_SN_on [of "{x}" g]
  by auto

lemma SN_on_conv_acc: "SN_on {(y, z). (z, y) \<in> r} {x} \<longleftrightarrow> x \<in> Wellfounded.acc r"
  unfolding SN_on_conv_accp accp_acc_eq ..

lemma acc_imp_SN_on:
  assumes "x \<in> Wellfounded.acc r" shows "SN_on {(y, z). (z, y) \<in> r} {x}"
  using assms unfolding SN_on_conv_acc by simp

lemma SN_on_imp_acc:
  assumes "SN_on {(y, z). (z, y) \<in> r} {x}" shows "x \<in> Wellfounded.acc r"
  using assms unfolding SN_on_conv_acc by simp


subsection \<open>Newman's Lemma\<close>

lemma rtrancl_len_E [elim]:
  assumes "(x, y) \<in> r\<^sup>*" obtains n where "(x, y) \<in> r^^n"
  using rtrancl_imp_UN_relpow [OF assms] by best

lemma relpow_Suc_E2' [elim]:
  assumes "(x, z) \<in> A^^Suc n" obtains y where "(x, y) \<in> A" and "(y, z) \<in> A\<^sup>*"
proof -
  assume assm: "\<And>y. (x, y) \<in> A \<Longrightarrow> (y, z) \<in> A\<^sup>* \<Longrightarrow> thesis"
  from relpow_Suc_E2 [OF assms] obtain y where "(x, y) \<in> A" and "(y, z) \<in> A^^n" by auto
  then have "(y, z) \<in> A\<^sup>*" using (*FIXME*) relpow_imp_rtrancl by auto
  from assm [OF \<open>(x, y) \<in> A\<close> this] show thesis .
qed

lemmas SN_on_induct' [consumes 1, case_names IH] = SN_on_induct [OF _ singletonI]

lemma Newman_local:
  assumes "SN_on r X" and WCR: "WCR_on r {x. SN_on r {x}}"
  shows "CR_on r X"
proof - {
  fix x
  assume "x \<in> X"
  with assms have "SN_on r {x}" unfolding SN_on_def by auto
  with this  have "CR_on r {x}"
  proof (induct rule: SN_on_induct')
    case (IH x) show ?case
    proof
      fix y z assume "(x, y) \<in> r\<^sup>*" and "(x, z) \<in> r\<^sup>*"
      from \<open>(x, y) \<in> r\<^sup>*\<close> obtain m where "(x, y) \<in> r^^m" ..
      from \<open>(x, z) \<in> r\<^sup>*\<close> obtain n where "(x, z) \<in> r^^n" ..
      show "(y, z) \<in> r\<^sup>\<down>"
      proof (cases n)
        case 0
        from \<open>(x, z) \<in> r^^n\<close> have eq: "x = z" by (simp add: 0)
        from \<open>(x, y) \<in> r\<^sup>*\<close> show ?thesis unfolding eq ..
      next
        case (Suc n')
        from \<open>(x, z) \<in> r^^n\<close> [unfolded Suc] obtain t where "(x, t) \<in> r" and "(t, z) \<in> r\<^sup>*" ..
        show ?thesis
        proof (cases m)
          case 0
          from \<open>(x, y) \<in> r^^m\<close> have eq: "x = y" by (simp add: 0)
          from \<open>(x, z) \<in> r\<^sup>*\<close> show ?thesis unfolding eq ..
        next
          case (Suc m')
          from \<open>(x, y) \<in> r^^m\<close> [unfolded Suc] obtain s where "(x, s) \<in> r" and "(s, y) \<in> r\<^sup>*" ..          
          from WCR IH(2) have "WCR_on r {x}" unfolding WCR_on_def by auto
          with \<open>(x, s) \<in> r\<close> and \<open>(x, t) \<in> r\<close> have "(s, t) \<in> r\<^sup>\<down>" by auto
          then obtain u where "(s, u) \<in> r\<^sup>*" and "(t, u) \<in> r\<^sup>*" ..
          from \<open>(x, s) \<in> r\<close> IH(2) have "SN_on r {s}" by (rule step_preserves_SN_on)
          from IH(1)[OF \<open>(x, s) \<in> r\<close> this] have "CR_on r {s}" .
          from this and \<open>(s, u) \<in> r\<^sup>*\<close> and \<open>(s, y) \<in> r\<^sup>*\<close> have "(u, y) \<in> r\<^sup>\<down>" by auto
          then obtain v where "(u, v) \<in> r\<^sup>*" and "(y, v) \<in> r\<^sup>*" ..
          from \<open>(x, t) \<in> r\<close> IH(2) have "SN_on r {t}" by (rule step_preserves_SN_on)
          from IH(1)[OF \<open>(x, t) \<in> r\<close> this] have "CR_on r {t}" .
          moreover from \<open>(t, u) \<in> r\<^sup>*\<close> and \<open>(u, v) \<in> r\<^sup>*\<close> have "(t, v) \<in> r\<^sup>*" by auto
          ultimately have "(z, v) \<in> r\<^sup>\<down>" using \<open>(t, z) \<in> r\<^sup>*\<close> by auto
          then obtain w where "(z, w) \<in> r\<^sup>*" and "(v, w) \<in> r\<^sup>*" ..
          from \<open>(y, v) \<in> r\<^sup>*\<close> and \<open>(v, w) \<in> r\<^sup>*\<close> have "(y, w) \<in> r\<^sup>*" by auto
          with \<open>(z, w) \<in> r\<^sup>*\<close> show ?thesis by auto
        qed
      qed
    qed
  qed
  }
  then show ?thesis unfolding CR_on_def by blast
qed

lemma Newman: "SN r \<Longrightarrow> WCR r \<Longrightarrow> CR r"
  using Newman_local [of r UNIV]
  unfolding WCR_on_def by auto
  
lemma Image_SN_on:
  assumes "SN_on r (r `` A)"
  shows "SN_on r A"
proof
  fix f
  assume "f 0 \<in> A" and chain: "chain r f"
  then have "f (Suc 0) \<in> r `` A" by auto
  with assms have "SN_on r {f (Suc 0)}" by (auto simp add: \<open>f 0 \<in> A\<close> SN_defs)
  moreover have "\<not> SN_on r {f (Suc 0)}"
  proof -
    have "f (Suc 0) \<in> {f (Suc 0)}" by simp
    moreover from chain have "chain r (f \<circ> Suc)" by auto
    ultimately show ?thesis by auto
  qed
  ultimately show False by simp
qed

lemma SN_on_Image_conv: "SN_on r (r `` A) = SN_on r A"
  using SN_on_Image and Image_SN_on by blast

text \<open>If all successors are terminating, then the current element is also terminating.\<close>
lemma step_reflects_SN_on:
  assumes "(\<And>b. (a, b) \<in> r \<Longrightarrow> SN_on r {b})"
  shows "SN_on r {a}"
  using assms and Image_SN_on [of r "{a}"] by (auto simp: SN_defs)

lemma SN_on_all_reducts_SN_on_conv:
  "SN_on r {a} = (\<forall>b. (a, b) \<in> r \<longrightarrow> SN_on r {b})"
  using SN_on_Image_conv [of r "{a}"] by (auto simp: SN_defs)

lemma SN_imp_SN_trancl: "SN R \<Longrightarrow> SN (R\<^sup>+)"
  unfolding SN_iff_wf by (rule wf_converse_trancl)

lemma SN_trancl_imp_SN:
  assumes "SN (R\<^sup>+)" shows "SN R"
  using assms by (rule SN_on_trancl_imp_SN_on)

lemma SN_trancl_SN_conv: "SN (R\<^sup>+) = SN R"
  using SN_trancl_imp_SN [of R] SN_imp_SN_trancl [of R] by blast

lemma SN_inv_image: "SN R \<Longrightarrow> SN (inv_image R f)" unfolding SN_iff_wf by simp

lemma SN_subset: "SN R \<Longrightarrow> R' \<subseteq> R \<Longrightarrow> SN R'" unfolding SN_defs by blast
 
lemma SN_pow_imp_SN:
  assumes "SN (A^^Suc n)" shows "SN A"
proof (rule ccontr)
  assume "\<not> SN A"
  then obtain S where "chain A S" unfolding SN_defs by auto
  from chain_imp_relpow [OF this]
    have step: "\<And>i. (S i, S (i + (Suc n))) \<in> A^^Suc n" .
  let ?T = "\<lambda>i. S (i * (Suc n))"
  have "chain (A^^Suc n) ?T"
  proof
    fix i show "(?T i, ?T (Suc i)) \<in> A^^Suc n" unfolding mult_Suc
      using step [of "i * Suc n"] by (simp only: add.commute)
  qed
  then have "\<not> SN (A^^Suc n)" unfolding SN_defs by fast
  with assms show False by simp
qed

(* TODO: move to Isabelle Library? *)
lemma pow_Suc_subset_trancl: "R^^(Suc n) \<subseteq> R\<^sup>+"
  using trancl_power [of _ R] by blast

lemma SN_imp_SN_pow:
  assumes "SN R" shows "SN (R^^Suc n)"
  using SN_subset [where R="R\<^sup>+", OF SN_imp_SN_trancl [OF assms] pow_Suc_subset_trancl] by simp
  
(*FIXME: needed in HOL/Wellfounded.thy*)
lemma SN_pow: "SN R \<longleftrightarrow> SN (R ^^ Suc n)"
  by (rule iffI, rule SN_imp_SN_pow, assumption, rule SN_pow_imp_SN, assumption)

lemma SN_on_trancl:
  assumes "SN_on r A" shows "SN_on (r\<^sup>+) A"
using assms
proof (rule contrapos_pp)
  let ?r = "restrict_SN r r"
  assume "\<not> SN_on (r\<^sup>+) A"
  then obtain f where "f 0 \<in> A" and chain: "chain (r\<^sup>+) f" by auto
  have "SN ?r" by (rule SN_restrict_SN_idemp)
  then have "SN (?r\<^sup>+)" by (rule SN_imp_SN_trancl)
  have "\<forall>i. (f 0, f i) \<in> r\<^sup>*"
  proof
    fix i show "(f 0, f i) \<in> r\<^sup>*"
    proof (induct i)
      case 0 show ?case ..
    next
      case (Suc i)
      from chain have "(f i, f (Suc i)) \<in> r\<^sup>+" ..
      with Suc show ?case by auto
    qed
  qed
  with assms have "\<forall>i. SN_on r {f i}"
    using steps_preserve_SN_on [of "f 0" _ r]
    and \<open>f 0 \<in> A\<close>
    and SN_on_subset2 [of "{f 0}" "A"] by auto
  with chain have "chain (?r\<^sup>+) f"
    unfolding restrict_SN_trancl_simp
    unfolding restrict_SN_def by auto
  then have "\<not> SN_on (?r\<^sup>+) {f 0}" by auto
  with \<open>SN (?r\<^sup>+)\<close> have False by (simp add: SN_defs)
  then show "\<not> SN_on r A" by simp
qed

lemma SN_on_trancl_SN_on_conv: "SN_on (R\<^sup>+) T = SN_on R T"
  using SN_on_trancl_imp_SN_on [of R] SN_on_trancl [of R] by blast


text \<open>Restrict an ARS to elements of a given set.\<close>
definition "restrict" :: "'a rel \<Rightarrow> 'a set \<Rightarrow> 'a rel" where
  "restrict r S = {(x, y). x \<in> S \<and> y \<in> S \<and> (x, y) \<in> r}"

lemma SN_on_restrict:
  assumes "SN_on r A"
  shows "SN_on (restrict r S) A" (is "SN_on ?r A")
proof (rule ccontr)
  assume "\<not> SN_on ?r A"
  then have "\<exists>f. f 0 \<in> A \<and> chain ?r f" by auto
  then have "\<exists>f. f 0 \<in> A \<and> chain r f" unfolding restrict_def by auto
  with \<open>SN_on r A\<close> show False by auto
qed

lemma restrict_rtrancl: "(restrict r S)\<^sup>* \<subseteq> r\<^sup>*" (is "?r\<^sup>* \<subseteq> r\<^sup>*")
proof - {
  fix x y assume "(x, y) \<in> ?r\<^sup>*" then have "(x, y) \<in> r\<^sup>*" unfolding restrict_def by induct auto
} then show ?thesis by auto
qed

lemma rtrancl_Image_step:
  assumes "a \<in> r\<^sup>* `` A"
    and "(a, b) \<in> r\<^sup>*"
  shows "b \<in> r\<^sup>* `` A"
proof -
  from assms(1) obtain c where "c \<in> A" and "(c, a) \<in> r\<^sup>*" by auto
  with assms have "(c, b) \<in> r\<^sup>*" by auto
  with \<open>c \<in> A\<close> show ?thesis by auto
qed

lemma WCR_SN_on_imp_CR_on:
  assumes "WCR r" and "SN_on r A" shows "CR_on r A"
proof -
  let ?S = "r\<^sup>* `` A"
  let ?r = "restrict r ?S"
  have "\<forall>x. SN_on ?r {x}"
  proof
    fix y have "y \<notin> ?S \<or> y \<in> ?S" by simp
    then show "SN_on ?r {y}"
    proof
      assume "y \<notin> ?S" then show ?thesis unfolding restrict_def by auto
    next
      assume "y \<in> ?S"
      then have "y \<in> r\<^sup>* `` A" by simp
      with SN_on_Image_rtrancl [OF \<open>SN_on r A\<close>]
        have "SN_on r {y}" using SN_on_subset2 [of "{y}" "r\<^sup>* `` A"] by blast
      then show ?thesis by (rule SN_on_restrict)
    qed
  qed
  then have "SN ?r" unfolding SN_defs by auto
  {
    fix x y assume "(x, y) \<in> r\<^sup>*" and "x \<in> ?S" and "y \<in> ?S"
    then obtain n where "(x, y) \<in> r^^n" and "x \<in> ?S" and "y \<in> ?S"
      using rtrancl_imp_UN_relpow by best
    then have "(x, y) \<in> ?r\<^sup>*"
    proof (induct n arbitrary: x y)
      case 0 then show ?case by simp
    next
      case (Suc n)
      from \<open>(x, y) \<in> r^^Suc n\<close> obtain x' where "(x, x') \<in> r" and "(x', y) \<in> r^^n"
        using relpow_Suc_D2 by best
      then have "(x, x') \<in> r\<^sup>*" by simp
      with \<open>x \<in> ?S\<close> have "x' \<in> ?S" by (rule rtrancl_Image_step)
      with Suc and \<open>(x', y) \<in> r^^n\<close> have "(x', y) \<in> ?r\<^sup>*" by simp
      from \<open>(x, x') \<in> r\<close> and \<open>x \<in> ?S\<close> and \<open>x' \<in> ?S\<close> have "(x, x') \<in> ?r"
        unfolding restrict_def by simp
      with \<open>(x', y) \<in> ?r\<^sup>*\<close> show ?case by simp
    qed
  }
  then have a:"\<forall>x y. (x, y) \<in> r\<^sup>* \<and> x \<in> ?S \<and> y \<in> ?S \<longrightarrow> (x, y) \<in> ?r\<^sup>*" by simp
  {
    fix x' y z assume "(x', y) \<in> ?r" and "(x', z) \<in> ?r"
    then have "x' \<in> ?S" and "y \<in> ?S" and "z \<in> ?S" and "(x', y) \<in> r" and "(x', z) \<in> r"
      unfolding restrict_def by auto
    with \<open>WCR r\<close> have "(y, z) \<in> r\<^sup>\<down>" by auto
    then obtain u where "(y, u) \<in> r\<^sup>*" and "(z, u) \<in> r\<^sup>*" by auto
    from \<open>x' \<in> ?S\<close> obtain x where "x \<in> A" and "(x, x') \<in> r\<^sup>*" by auto
    from \<open>(x', y) \<in> r\<close> have "(x', y) \<in> r\<^sup>*" by auto
    with \<open>(y, u) \<in> r\<^sup>*\<close> have "(x', u) \<in> r\<^sup>*" by auto
    with \<open>(x, x') \<in> r\<^sup>*\<close> have "(x, u) \<in> r\<^sup>*" by simp
    then have "u \<in> ?S" using \<open>x \<in> A\<close> by auto
    from \<open>y \<in> ?S\<close> and \<open>u \<in> ?S\<close> and \<open>(y, u) \<in> r\<^sup>*\<close> have "(y, u) \<in> ?r\<^sup>*" using a by auto
    from \<open>z \<in> ?S\<close> and \<open>u \<in> ?S\<close> and \<open>(z, u) \<in> r\<^sup>*\<close> have "(z, u) \<in> ?r\<^sup>*" using a by auto
    with \<open>(y, u) \<in> ?r\<^sup>*\<close> have "(y, z) \<in> ?r\<^sup>\<down>" by auto
  }
  then have "WCR ?r" by auto
  have "CR ?r" using Newman [OF \<open>SN ?r\<close> \<open>WCR ?r\<close>] by simp
  {
    fix x y z assume "x \<in> A" and "(x, y) \<in> r\<^sup>*" and "(x, z) \<in> r\<^sup>*"
    then have "y \<in> ?S" and "z \<in> ?S" by auto
    have "x \<in> ?S" using \<open>x \<in> A\<close> by auto
    from a and \<open>(x, y) \<in> r\<^sup>*\<close> and \<open>x \<in> ?S\<close> and \<open>y \<in> ?S\<close> have "(x, y) \<in> ?r\<^sup>*" by simp
    from a and \<open>(x, z) \<in> r\<^sup>*\<close> and \<open>x \<in> ?S\<close> and \<open>z \<in> ?S\<close> have "(x, z) \<in> ?r\<^sup>*" by simp
    with \<open>CR ?r\<close> and \<open>(x, y) \<in> ?r\<^sup>*\<close> have "(y, z) \<in> ?r\<^sup>\<down>" by auto
    then obtain u where "(y, u) \<in> ?r\<^sup>*" and "(z, u) \<in> ?r\<^sup>*" by best
    then have "(y, u) \<in> r\<^sup>*" and "(z, u) \<in> r\<^sup>*" using restrict_rtrancl by auto
    then have "(y, z) \<in> r\<^sup>\<down>" by auto
  }
  then show ?thesis by auto
qed

lemma SN_on_Image_normalizable:
  assumes "SN_on r A"
  shows "\<forall>a\<in>A. \<exists>b. b \<in> r\<^sup>! `` A"
proof
  fix a assume a: "a \<in> A"
  show "\<exists>b. b \<in> r\<^sup>! `` A"
  proof (rule ccontr)
    assume "\<not> (\<exists>b. b \<in> r\<^sup>! `` A)"
    then have A: "\<forall>b. (a, b) \<in> r\<^sup>* \<longrightarrow> b \<notin> NF r" using a by auto
    then have "a \<notin> NF r" by auto
    let ?Q = "{c. (a, c) \<in> r\<^sup>* \<and> c \<notin> NF r}"
    have "a \<in> ?Q" using \<open>a \<notin> NF r\<close> by simp
    have "\<forall>c\<in>?Q. \<exists>b. (c, b) \<in> r \<and> b \<in> ?Q"
    proof
      fix c
      assume "c \<in> ?Q"
      then have "(a, c) \<in> r\<^sup>*" and "c \<notin> NF r" by auto
      then obtain d where "(c, d) \<in> r" by auto
      with \<open>(a, c) \<in> r\<^sup>*\<close> have "(a, d) \<in> r\<^sup>*" by simp
      with A have "d \<notin> NF r" by simp
      with \<open>(c, d) \<in> r\<close> and \<open>(a, c) \<in> r\<^sup>*\<close>
        show "\<exists>b. (c, b) \<in> r \<and> b \<in> ?Q" by auto
    qed
    with \<open>a \<in> ?Q\<close> have "a \<in> ?Q \<and> (\<forall>c\<in>?Q. \<exists>b. (c, b) \<in> r \<and> b \<in> ?Q)" by auto
    then have "\<exists>Q. a \<in> Q \<and> (\<forall>c\<in>Q. \<exists>b. (c, b) \<in> r \<and> b \<in> Q)" by (rule exI [of _ "?Q"])
    then have "\<not> (\<forall>Q. a \<in> Q \<longrightarrow> (\<exists>c\<in>Q. \<forall>b. (c, b) \<in> r \<longrightarrow> b \<notin> Q))" by simp
    with SN_on_imp_on_minimal [of r a] have "\<not> SN_on r {a}" by blast
    with assms and \<open>a \<in> A\<close> and SN_on_subset2 [of "{a}" A r] show False by simp
  qed
qed

lemma SN_on_imp_normalizability:
  assumes "SN_on r {a}" shows "\<exists>b. (a, b) \<in> r\<^sup>!"
  using SN_on_Image_normalizable [OF assms] by auto


subsection \<open>Commutation\<close>

definition commute :: "'a rel \<Rightarrow> 'a rel \<Rightarrow> bool" where
  "commute r s \<longleftrightarrow> ((r\<inverse>)\<^sup>* O s\<^sup>*) \<subseteq> (s\<^sup>* O (r\<inverse>)\<^sup>*)"

lemma CR_iff_self_commute: "CR r = commute r r"
  unfolding commute_def CR_iff_meet_subset_join meet_def join_def
  by simp

(* FIXME: move somewhere else *)
lemma rtrancl_imp_rtrancl_UN: 
  assumes "(x, y) \<in> r\<^sup>*" and "r \<in> I"
  shows "(x, y) \<in> (\<Union>r\<in>I. r)\<^sup>*" (is "(x, y) \<in> ?r\<^sup>*")
using assms proof induct
  case base then show ?case by simp
next
  case (step y z)
  then have "(x, y) \<in> ?r\<^sup>*" by simp
  from \<open>(y, z) \<in> r\<close> and \<open>r \<in> I\<close> have "(y, z) \<in> ?r\<^sup>*" by auto
  with \<open>(x, y) \<in> ?r\<^sup>*\<close> show ?case by auto
qed

definition quasi_commute :: "'a rel \<Rightarrow> 'a rel \<Rightarrow> bool" where
  "quasi_commute r s \<longleftrightarrow> (s O r) \<subseteq> r O (r \<union> s)\<^sup>*"

lemma rtrancl_union_subset_rtrancl_union_trancl: "(r \<union> s\<^sup>+)\<^sup>* = (r \<union> s)\<^sup>*"
proof
  show "(r \<union> s\<^sup>+)\<^sup>* \<subseteq> (r \<union> s)\<^sup>*"
  proof (rule subrelI)
    fix x y assume "(x, y) \<in> (r \<union> s\<^sup>+)\<^sup>*"
    then show "(x, y) \<in> (r \<union> s)\<^sup>*"
    proof (induct)
      case base then show ?case by auto
    next
      case (step y z)
      then have "(y, z) \<in> r \<or> (y, z) \<in> s\<^sup>+" by auto
      then have "(y, z) \<in> (r \<union> s)\<^sup>*"
      proof
        assume "(y, z) \<in> r" then show ?thesis by auto
      next
        assume "(y, z) \<in> s\<^sup>+"
        then have "(y, z) \<in> s\<^sup>*" by auto
        then have "(y, z) \<in> r\<^sup>* \<union> s\<^sup>*" by auto
        then show ?thesis using rtrancl_Un_subset by auto
      qed
      with \<open>(x, y) \<in> (r \<union> s)\<^sup>*\<close> show ?case by simp
    qed
  qed
next
  show "(r \<union> s)\<^sup>* \<subseteq> (r \<union> s\<^sup>+)\<^sup>*"
  proof (rule subrelI)
    fix x y assume "(x, y) \<in> (r \<union> s)\<^sup>*"
    then show "(x, y) \<in> (r \<union> s\<^sup>+)\<^sup>*"
    proof (induct)
      case base then show ?case by auto
    next
      case (step y z)
      then have "(y, z) \<in> (r \<union> s\<^sup>+)\<^sup>*" by auto
      with \<open>(x, y) \<in> (r \<union> s\<^sup>+)\<^sup>*\<close> show ?case by auto
    qed
  qed
qed

lemma qc_imp_qc_trancl:
  assumes "quasi_commute r s" shows "quasi_commute r (s\<^sup>+)"
unfolding quasi_commute_def
proof (rule subrelI)
  fix x z assume "(x, z) \<in> s\<^sup>+ O r"
  then obtain y where "(x, y) \<in> s\<^sup>+" and "(y, z) \<in> r" by best
  then show "(x, z) \<in> r O (r \<union> s\<^sup>+)\<^sup>*"
  proof (induct arbitrary: z)
    case (base y)
    then have "(x, z) \<in> (s O r)" by auto
    with assms have "(x, z) \<in> r O (r \<union> s)\<^sup>*" unfolding quasi_commute_def by auto
    then show ?case using rtrancl_union_subset_rtrancl_union_trancl by auto
  next
    case (step a b)
    then have "(a, z) \<in> (s O r)" by auto
    with assms have "(a, z) \<in> r O (r \<union> s)\<^sup>*" unfolding quasi_commute_def by auto
    then obtain u where "(a, u) \<in> r" and "(u, z) \<in> (r \<union> s)\<^sup>*" by best
    then have "(u, z) \<in> (r \<union> s\<^sup>+)\<^sup>*" using rtrancl_union_subset_rtrancl_union_trancl by auto
    from \<open>(a, u) \<in> r\<close> and step have "(x, u) \<in> r O (r \<union> s\<^sup>+)\<^sup>*" by auto
    then obtain v where "(x, v) \<in> r" and "(v, u) \<in> (r \<union> s\<^sup>+)\<^sup>*" by best
    with \<open>(u, z) \<in> (r \<union> s\<^sup>+)\<^sup>*\<close> have "(v, z) \<in> (r \<union> s\<^sup>+)\<^sup>*" by auto
    with \<open>(x, v) \<in> r\<close> show ?case by auto
  qed
qed

lemma steps_reflect_SN_on:
  assumes "\<not> SN_on r {b}" and "(a, b) \<in> r\<^sup>*"
  shows "\<not> SN_on r {a}"
  using SN_on_Image_rtrancl [of r "{a}"]
  and assms and SN_on_subset2 [of "{b}" "r\<^sup>* `` {a}" r] by blast

lemma chain_imp_not_SN_on:
   assumes "chain r f"
   shows "\<not> SN_on r {f i}"
proof -
  let ?f = "\<lambda>j. f (i + j)"
  have "?f 0 \<in> {f i}" by simp
  moreover have "chain r ?f" using assms by auto
  ultimately have "?f 0 \<in> {f i} \<and> chain r ?f" by blast
  then have "\<exists>g. g 0 \<in> {f i} \<and> chain r g" by (rule exI [of _ "?f"])
  then show ?thesis unfolding SN_defs by auto
qed

lemma quasi_commute_imp_SN:
  assumes "SN r" and "SN s" and "quasi_commute r s"
  shows "SN (r \<union> s)"
proof -
  have "quasi_commute r (s\<^sup>+)" by (rule qc_imp_qc_trancl [OF \<open>quasi_commute r s\<close>])
  let ?B = "{a. \<not> SN_on (r \<union> s) {a}}"
  {
    assume "\<not> SN(r \<union> s)"
    then obtain a where "a \<in> ?B" unfolding SN_defs by fast
    from \<open>SN r\<close> have "\<forall>Q x. x \<in> Q \<longrightarrow> (\<exists>z\<in>Q. \<forall>y. (z, y) \<in> r \<longrightarrow> y \<notin> Q)"
      by (rule SN_imp_minimal)
    then have "\<forall>x. x \<in> ?B \<longrightarrow> (\<exists>z\<in>?B. \<forall>y. (z, y) \<in> r \<longrightarrow> y \<notin> ?B)" by (rule spec [where x = ?B])
    with \<open>a \<in> ?B\<close> obtain b where "b \<in> ?B" and min: "\<forall>y. (b, y) \<in> r \<longrightarrow> y \<notin> ?B" by auto
    from \<open>b \<in> ?B\<close> obtain S where "S 0 = b" and
      chain: "chain (r \<union> s) S" unfolding SN_on_def by auto
    let ?S = "\<lambda>i. S(Suc i)"
    have "?S 0 = S 1" by simp
    from chain have "chain (r \<union> s) ?S" by auto
    with \<open>?S 0 = S 1\<close> have "\<not> SN_on (r \<union> s) {S 1}" unfolding SN_on_def by auto
    from \<open>S 0 = b\<close> and chain have "(b, S 1) \<in> r \<union> s" by auto
    with min and \<open>\<not> SN_on (r \<union> s) {S 1}\<close> have "(b, S 1) \<in> s" by auto
    let ?i = "LEAST i. (S i, S(Suc i)) \<notin> s"
    {
      assume "chain s S"
      with \<open>S 0 = b\<close> have "\<not> SN_on s {b}" unfolding SN_on_def by auto
      with \<open>SN s\<close> have False unfolding SN_defs by auto
    }
    then have ex: "\<exists>i. (S i, S(Suc i)) \<notin> s" by auto
    then have "(S ?i, S(Suc ?i)) \<notin> s" by (rule LeastI_ex)
    with chain have "(S ?i, S(Suc ?i)) \<in> r" by auto
    have ini: "\<forall>i<?i. (S i, S(Suc i)) \<in> s" using not_less_Least by auto
    {
      fix i assume "i < ?i" then have "(b, S(Suc i)) \<in> s\<^sup>+"
      proof (induct i)
        case 0 then show ?case using \<open>(b, S 1) \<in> s\<close> and \<open>S 0 = b\<close> by auto
      next
        case (Suc k)
      then have "(b, S(Suc k)) \<in> s\<^sup>+" and "Suc k < ?i" by auto
      with \<open>\<forall>i<?i. (S i, S(Suc i)) \<in> s\<close> have "(S(Suc k), S(Suc(Suc k))) \<in> s" by fast
      with \<open>(b, S(Suc k)) \<in> s\<^sup>+\<close> show ?case by auto
    qed
    }
    then have pref: "\<forall>i<?i. (b, S(Suc i)) \<in> s\<^sup>+" by auto
    from \<open>(b, S 1) \<in> s\<close> and \<open>S 0 = b\<close> have "(S 0, S(Suc 0)) \<in> s" by auto
    {
      assume "?i = 0"
      from ex have "(S ?i, S(Suc ?i)) \<notin> s" by (rule LeastI_ex)
      with \<open>(S 0, S(Suc 0)) \<in> s\<close> have False unfolding \<open>?i = 0\<close> by simp
    }
    then have "0 < ?i" by auto
    then obtain j where "?i =  Suc j" unfolding gr0_conv_Suc by best
    with ini have "(S(?i-Suc 0), S(Suc(?i-Suc 0))) \<in> s" by auto
    with pref have "(b, S(Suc j)) \<in> s\<^sup>+" unfolding \<open>?i = Suc j\<close> by auto
    then have "(b, S ?i) \<in> s\<^sup>+" unfolding \<open>?i = Suc j\<close> by auto
    with \<open>(S ?i, S(Suc ?i)) \<in> r\<close> have "(b, S(Suc ?i)) \<in> (s\<^sup>+ O r)" by auto
    with \<open>quasi_commute r (s\<^sup>+)\<close> have "(b, S(Suc ?i)) \<in> r O (r \<union> s\<^sup>+)\<^sup>*"
      unfolding quasi_commute_def by auto
    then obtain c where "(b, c) \<in> r" and "(c, S(Suc ?i)) \<in> (r \<union> s\<^sup>+)\<^sup>*" by best
    from \<open>(b, c) \<in> r\<close> have "(b, c) \<in> (r \<union> s)\<^sup>*" by auto
    from chain_imp_not_SN_on [of S "r \<union> s"]
      and chain have "\<not> SN_on (r \<union> s) {S (Suc ?i)}" by auto
    from \<open>(c, S(Suc ?i)) \<in> (r \<union> s\<^sup>+)\<^sup>*\<close> have "(c, S(Suc ?i)) \<in> (r \<union> s)\<^sup>*"
      unfolding rtrancl_union_subset_rtrancl_union_trancl by auto
    with steps_reflect_SN_on [of "r \<union> s"]
      and \<open>\<not> SN_on (r \<union> s) {S(Suc ?i)}\<close> have "\<not> SN_on (r \<union> s) {c}" by auto
    then have "c \<in> ?B" by simp
    with \<open>(b, c) \<in> r\<close> and min have False by auto
  }
  then show ?thesis by auto
qed


subsection \<open>Strong Normalization\<close>

lemma non_strict_into_strict:
  assumes compat: "NS O S \<subseteq> S"
    and steps: "(s, t) \<in> (NS\<^sup>*) O S"
  shows "(s, t) \<in> S"
using steps proof
  fix x u z
  assume "(s, t) = (x, z)" and "(x, u) \<in> NS\<^sup>*" and "(u, z) \<in> S"
  then have "(s, u) \<in> NS\<^sup>*" and "(u, t) \<in> S" by auto
  then show ?thesis
  proof (induct rule:rtrancl.induct)
    case (rtrancl_refl x) then show ?case .
  next
    case (rtrancl_into_rtrancl a b c)
    with compat show ?case by auto
  qed
qed

lemma comp_trancl:
  assumes "R O S \<subseteq> S" shows "R O S\<^sup>+ \<subseteq> S\<^sup>+"
proof (rule subrelI)
  fix w z assume "(w, z) \<in> R O S\<^sup>+"
  then obtain x where R_step: "(w, x) \<in> R" and S_seq: "(x, z) \<in> S\<^sup>+" by best
  from tranclD [OF S_seq] obtain y where S_step: "(x, y) \<in> S" and S_seq': "(y, z) \<in> S\<^sup>*" by auto
  from R_step and S_step have "(w, y) \<in> R O S" by auto
  with assms have "(w, y) \<in> S" by auto
  with S_seq' show "(w, z) \<in> S\<^sup>+" by simp
qed

lemma comp_rtrancl_trancl:
  assumes comp: "R O S \<subseteq> S"
    and seq: "(s, t) \<in> (R \<union> S)\<^sup>* O S"
  shows "(s, t) \<in> S\<^sup>+"
using seq proof
  fix x u z
  assume "(s, t) = (x, z)" and "(x, u) \<in> (R \<union> S)\<^sup>*" and "(u, z) \<in> S"
  then have "(s, u) \<in> (R \<union> S)\<^sup>*" and "(u, t) \<in> S\<^sup>+" by auto
  then show ?thesis
  proof (induct rule: rtrancl.induct)
    case (rtrancl_refl x) then show ?case .
  next
    case (rtrancl_into_rtrancl a b c)
    then have "(b, c) \<in> R \<union> S" by simp
    then show ?case
    proof
      assume "(b, c) \<in> S"
      with rtrancl_into_rtrancl
      have "(b, t) \<in> S\<^sup>+" by simp
      with rtrancl_into_rtrancl show ?thesis by simp
    next
      assume "(b, c) \<in> R"
      with comp_trancl [OF comp] rtrancl_into_rtrancl
      show ?thesis by auto
    qed
  qed
qed

lemma trancl_union_right: "r\<^sup>+ \<subseteq> (s \<union> r)\<^sup>+"
proof (rule subrelI)
  fix x y assume "(x, y) \<in> r\<^sup>+" then show "(x, y) \<in> (s \<union> r)\<^sup>+"
  proof (induct)
    case base then show ?case by auto
  next
    case (step a b)
    then have "(a, b) \<in> (s \<union> r)\<^sup>+" by auto
    with \<open>(x, a) \<in> (s \<union> r)\<^sup>+\<close> show ?case by auto
  qed
qed

lemma restrict_SN_subset: "restrict_SN R S \<subseteq> R"
proof (rule subrelI)
  fix a b assume "(a, b) \<in> restrict_SN R S" then show "(a, b) \<in> R" unfolding restrict_SN_def by simp
qed

lemma chain_Un_SN_on_imp_first_step:
  assumes "chain (R \<union> S) t" and "SN_on S {t 0}"
  shows "\<exists>i. (t i, t (Suc i)) \<in> R \<and> (\<forall>j<i. (t j, t (Suc j)) \<in> S \<and> (t j, t (Suc j)) \<notin> R)"
proof -
  from \<open>SN_on S {t 0}\<close> obtain i where "(t i, t (Suc i)) \<notin> S" by blast
  with assms have "(t i, t (Suc i)) \<in> R" (is "?P i") by auto
  let ?i = "Least ?P"
  from \<open>?P i\<close> have "?P ?i" by (rule LeastI)
  have "\<forall>j<?i. (t j, t (Suc j)) \<notin> R" using not_less_Least by auto
  moreover with assms have "\<forall>j<?i. (t j, t (Suc j)) \<in> S" by best
  ultimately have "\<forall>j<?i. (t j, t (Suc j)) \<in> S \<and> (t j, t (Suc j)) \<notin> R" by best
  with \<open>?P ?i\<close> show ?thesis by best
qed

lemma first_step:
  assumes C: "C = A \<union> B" and steps: "(x, y) \<in> C\<^sup>*" and Bstep: "(y, z) \<in> B"
  shows "\<exists>y. (x, y) \<in> A\<^sup>* O B"
  using steps
proof (induct rule: converse_rtrancl_induct)
  case base
  show ?case using Bstep by auto
next 
  case (step u x)
  from step(1)[unfolded C] 
  show ?case
  proof
    assume "(u, x) \<in> B"
    then show ?thesis by auto
  next
    assume ux: "(u, x) \<in> A"
    from step(3) obtain y where "(x, y) \<in> A\<^sup>* O B" by auto
    then obtain z where "(x, z) \<in> A\<^sup>*" and step: "(z, y) \<in> B" by auto
    with ux have "(u, z) \<in> A\<^sup>*" by auto
    with step have "(u, y) \<in> A\<^sup>* O B" by auto
    then show ?thesis by auto
  qed
qed

lemma first_step_O:
  assumes C: "C = A \<union> B" and steps: "(x, y) \<in> C\<^sup>* O B"
  shows "\<exists> y. (x, y) \<in> A\<^sup>* O B"
proof -
  from steps obtain z where "(x, z) \<in> C\<^sup>*" and "(z, y) \<in> B" by auto
  from first_step [OF C this] show ?thesis .
qed

lemma firstStep:
  assumes LSR: "L = S \<union> R" and xyL: "(x, y) \<in> L\<^sup>*"
  shows "(x, y) \<in> R\<^sup>* \<or> (x, y) \<in> R\<^sup>* O S O L\<^sup>*"
proof (cases "(x, y) \<in> R\<^sup>*")
  case True
  then show ?thesis by simp
next
  case False 
  let ?SR = "S \<union> R"
  from xyL and LSR have "(x, y) \<in> ?SR\<^sup>*" by simp
  from this and False have "(x, y) \<in> R\<^sup>* O S O ?SR\<^sup>*" 
  proof (induct rule: rtrancl_induct)
    case base then show ?case by simp
  next
    case (step y z)
    then show ?case
    proof (cases "(x, y) \<in> R\<^sup>*")
      case False with step have "(x, y) \<in> R\<^sup>* O S O ?SR\<^sup>*" by simp
      from this obtain u where xu: "(x, u) \<in> R\<^sup>* O S" and uy: "(u, y) \<in> ?SR\<^sup>*" by force
      from \<open>(y, z) \<in> ?SR\<close> have "(y, z) \<in> ?SR\<^sup>*" by auto
      with uy have "(u, z) \<in> ?SR\<^sup>*" by (rule rtrancl_trans)
      with xu show ?thesis by auto
    next
      case True 
      have "(y, z) \<in> S" 
      proof (rule ccontr)
        assume "(y, z) \<notin> S" with \<open>(y, z) \<in> ?SR\<close> have "(y, z) \<in> R" by auto
        with True  have "(x, z) \<in> R\<^sup>*"  by auto
        with \<open>(x, z) \<notin> R\<^sup>*\<close> show False ..
      qed
      with True show ?thesis by auto
    qed
  qed
  with LSR show ?thesis by simp
qed


lemma non_strict_ending:
  assumes chain: "chain (R \<union> S) t"
    and comp: "R O S \<subseteq> S"
    and SN: "SN_on S {t 0}"
  shows "\<exists>j. \<forall>i\<ge>j. (t i, t (Suc i)) \<in> R - S"
proof (rule ccontr)
  assume "\<not> ?thesis"
  with chain have "\<forall>i. \<exists>j. j \<ge> i \<and> (t j, t (Suc j)) \<in> S" by blast
  from choice [OF this] obtain f where S_steps: "\<forall>i. i \<le> f i \<and> (t (f i), t (Suc (f i))) \<in> S" ..
  let ?t = "\<lambda>i. t (((Suc \<circ> f) ^^ i) 0)"
  have S_chain: "\<forall>i. (t i, t (Suc (f i))) \<in> S\<^sup>+"
  proof
    fix i
    from S_steps have leq: "i\<le>f i" and step: "(t(f i), t(Suc(f i))) \<in> S" by auto
    from chain_imp_rtrancl [OF chain leq] have "(t i, t(f i)) \<in> (R \<union> S)\<^sup>*" .
    with step have "(t i, t(Suc(f i))) \<in> (R \<union> S)\<^sup>* O S" by auto
    from comp_rtrancl_trancl [OF comp this] show "(t i, t(Suc(f i))) \<in> S\<^sup>+" .
  qed
  then have "chain (S\<^sup>+) ?t"by simp
  moreover have "SN_on (S\<^sup>+) {?t 0}" using SN_on_trancl [OF SN] by simp
  ultimately show False unfolding SN_defs by best
qed

lemma SN_on_subset1:
  assumes "SN_on r A" and "s \<subseteq> r"
  shows "SN_on s A"
  using assms unfolding SN_defs by blast

lemmas SN_on_mono = SN_on_subset1

lemma rtrancl_fun_conv:
  "((s, t) \<in> R\<^sup>*) = (\<exists> f n. f 0 = s \<and> f n = t \<and> (\<forall> i < n. (f i, f (Suc i)) \<in> R))"
  unfolding rtrancl_is_UN_relpow using relpow_fun_conv [where R = R]
  by auto

lemma compat_tr_compat:
  assumes "NS O S \<subseteq> S" shows "NS\<^sup>* O S \<subseteq> S"
  using non_strict_into_strict [where S = S and NS = NS] assms by blast

lemma right_comp_S [simp]:
  assumes "(x, y) \<in> S O (S O S\<^sup>* O NS\<^sup>* \<union> NS\<^sup>*)"
  shows "(x, y) \<in> (S O S\<^sup>* O NS\<^sup>*)"
proof-
  from assms have "(x, y) \<in> (S O S O S\<^sup>* O NS\<^sup>*) \<union> (S O NS\<^sup>*)" by auto
  then have  xy:"(x, y) \<in> (S O (S O S\<^sup>*) O NS\<^sup>*) \<union> (S O NS\<^sup>*)" by auto
  have "S O S\<^sup>* \<subseteq> S\<^sup>*" by auto
  with xy have "(x, y) \<in> (S O S\<^sup>* O NS\<^sup>*) \<union> (S O NS\<^sup>*)" by auto
  then show "(x, y) \<in> (S O S\<^sup>* O NS\<^sup>*)" by auto
qed

lemma compatible_SN:
  assumes SN: "SN S"
  and compat: "NS O S \<subseteq> S" 
  shows "SN (S O S\<^sup>* O NS\<^sup>*)" (is "SN ?A")
proof
  fix F assume chain: "chain ?A F"
  from compat compat_tr_compat have tr_compat: "NS\<^sup>* O S \<subseteq> S" by blast
  have "\<forall>i. (\<exists>y z. (F i, y) \<in> S \<and> (y, z)  \<in> S\<^sup>* \<and> (z, F (Suc i)) \<in> NS\<^sup>*)"
  proof
    fix i
    from chain have "(F i, F (Suc i)) \<in> (S O S\<^sup>* O NS\<^sup>*)" by auto
    then show "\<exists> y z. (F i, y)  \<in> S \<and> (y, z)  \<in> S\<^sup>* \<and> (z, F (Suc i)) \<in> NS\<^sup>*"
      unfolding relcomp_def (*FIXME:relcomp_unfold*) using mem_Collect_eq by auto
  qed
  then have "\<exists> f. (\<forall> i. (\<exists> z. (F i, f i)  \<in> S \<and> ((f i, z)  \<in> S\<^sup>*) \<and>(z, F (Suc i)) \<in> NS\<^sup>*))"
    by (rule choice)
  then obtain f
    where "\<forall> i. (\<exists> z. (F i, f i)  \<in> S \<and> ((f i, z)  \<in> S\<^sup>*) \<and>(z, F (Suc i)) \<in> NS\<^sup>*)" ..
  then have "\<exists> g. \<forall> i. (F i, f i)  \<in> S \<and> (f i, g i)  \<in> S\<^sup>* \<and> (g i, F (Suc i)) \<in> NS\<^sup>*"
    by (rule choice)
  then obtain g where "\<forall> i. (F i, f i)  \<in> S \<and> (f i, g i)  \<in> S\<^sup>* \<and> (g i, F (Suc i)) \<in> NS\<^sup>*" ..
  then have "\<forall> i. (f i, g i)  \<in> S\<^sup>* \<and> (g i, F (Suc i)) \<in> NS\<^sup>* \<and> (F (Suc i), f (Suc i))  \<in> S"
    by auto
  then have "\<forall> i. (f i, g i)  \<in> S\<^sup>* \<and> (g i, f (Suc i))  \<in> S" unfolding relcomp_def (*FIXME*)
    using tr_compat by auto
  then have all:"\<forall> i. (f i, g i)  \<in> S\<^sup>* \<and> (g i, f (Suc i))  \<in> S\<^sup>+" by auto
  have "\<forall> i. (f i, f (Suc i))  \<in> S\<^sup>+"
  proof
    fix i
    from all have "(f i, g i)  \<in> S\<^sup>* \<and> (g i, f (Suc i))  \<in> S\<^sup>+" ..
    then show "(f i, f (Suc i))  \<in> S\<^sup>+" using transitive_closure_trans by auto
  qed
  then have "\<exists>x. f 0 = x \<and> chain (S\<^sup>+) f"by auto
  then obtain x where "f 0 = x \<and> chain (S\<^sup>+) f" by auto
  then have "\<exists>f. f 0 = x \<and> chain (S\<^sup>+) f" by auto
  then have "\<not> SN_on (S\<^sup>+) {x}" by auto
  then have "\<not> SN (S\<^sup>+)" unfolding SN_defs by auto
  then have wfSconv:"\<not> wf ((S\<^sup>+)\<inverse>)" using SN_iff_wf by auto
  from SN have "wf (S\<inverse>)" using SN_imp_wf [where?r=S] by simp
  with wf_converse_trancl wfSconv show False by auto
qed

lemma compatible_rtrancl_split:
  assumes compat: "NS O S \<subseteq> S"
   and steps: "(x, y) \<in> (NS \<union> S)\<^sup>*"
  shows "(x, y) \<in> S O S\<^sup>* O NS\<^sup>* \<union> NS\<^sup>*"
proof-
  from steps have "\<exists> n. (x, y) \<in> (NS \<union> S)^^n" using rtrancl_imp_relpow [where ?R="NS \<union> S"] by auto
  then obtain n where "(x, y) \<in> (NS \<union> S)^^n" by auto
  then show "(x, y) \<in>  S O S\<^sup>* O NS\<^sup>* \<union> NS\<^sup>*"
  proof (induct n arbitrary: x, simp)
    case (Suc m)
    assume "(x, y) \<in> (NS \<union> S)^^(Suc m)"
    then have "\<exists> z. (x, z) \<in> (NS \<union> S) \<and> (z, y) \<in> (NS \<union> S)^^m"
      using relpow_Suc_D2 [where ?R="NS \<union> S"] by auto
    then obtain z where xz:"(x, z) \<in> (NS \<union> S)" and zy:"(z, y) \<in> (NS \<union> S)^^m" by auto
    with Suc have zy:"(z, y) \<in>  S O S\<^sup>* O NS\<^sup>* \<union> NS\<^sup>*" by auto
    then show "(x, y) \<in>  S O S\<^sup>* O NS\<^sup>* \<union> NS\<^sup>*"
    proof (cases "(x, z) \<in> NS")
      case True
      from compat compat_tr_compat have trCompat: "NS\<^sup>* O S \<subseteq> S" by blast
      from zy True have "(x, y) \<in> (NS O S O S\<^sup>* O NS\<^sup>*) \<union> (NS O NS\<^sup>*)" by auto
      then have "(x, y) \<in> ((NS O S) O S\<^sup>* O NS\<^sup>*) \<union> (NS O NS\<^sup>*)" by auto
      then have "(x, y) \<in> ((NS\<^sup>* O S) O S\<^sup>* O NS\<^sup>*) \<union> (NS O NS\<^sup>*)" by auto
      with trCompat have xy:"(x, y) \<in> (S O S\<^sup>* O NS\<^sup>*) \<union> (NS O NS\<^sup>*)" by auto
      have "NS O NS\<^sup>* \<subseteq> NS\<^sup>*" by auto
      with xy show "(x, y) \<in> (S O S\<^sup>* O NS\<^sup>*) \<union> NS\<^sup>*" by auto
    next
      case False
      with xz have xz:"(x, z) \<in> S" by auto
      with zy have "(x, y) \<in> S O (S O S\<^sup>* O NS\<^sup>* \<union> NS\<^sup>*)" by auto
      then show "(x, y) \<in> (S O S\<^sup>* O NS\<^sup>*) \<union> NS\<^sup>*" using right_comp_S by simp
    qed
  qed
qed

lemma compatible_conv:
  assumes compat: "NS O S \<subseteq> S" 
  shows "(NS \<union> S)\<^sup>* O S O (NS \<union> S)\<^sup>* = S O S\<^sup>* O NS\<^sup>*" 
proof -
  let ?NSuS = "NS \<union> S"
  let ?NSS = "S O S\<^sup>* O NS\<^sup>*"
  let ?midS = "?NSuS\<^sup>* O S O ?NSuS\<^sup>*"
  have one: "?NSS \<subseteq> ?midS" by regexp 
  have "?NSuS\<^sup>* O S \<subseteq> (?NSS \<union> NS\<^sup>*) O S"
    using compatible_rtrancl_split [where S = S and NS = NS] compat by blast
  also have "\<dots> \<subseteq> ?NSS O S \<union> NS\<^sup>* O S" by auto
  also have "\<dots> \<subseteq> ?NSS O S \<union> S" using compat compat_tr_compat [where S = S and NS = NS] by auto
  also have "\<dots> \<subseteq> S O ?NSuS\<^sup>*" by regexp
  finally have "?midS \<subseteq> S O ?NSuS\<^sup>* O ?NSuS\<^sup>*" by blast
  also have "\<dots> \<subseteq> S O ?NSuS\<^sup>*" by regexp
  also have "\<dots> \<subseteq> S O (?NSS \<union> NS\<^sup>*)"
    using compatible_rtrancl_split [where S = S and NS = NS] compat by blast
  also have "\<dots> \<subseteq> ?NSS" by regexp
  finally have two: "?midS \<subseteq> ?NSS" . 
  from one two show ?thesis by auto 
qed

lemma compatible_SN': 
  assumes compat: "NS O S \<subseteq> S" and SN: "SN S"
  shows "SN((NS \<union> S)\<^sup>* O S O (NS \<union> S)\<^sup>*)"
using compatible_conv [where S = S and NS = NS] 
  compatible_SN [where S = S and NS = NS] assms by force

lemma rtrancl_diff_decomp:
  assumes "(x, y) \<in> A\<^sup>* - B\<^sup>*"
  shows "(x, y) \<in> A\<^sup>* O (A - B) O A\<^sup>*"
proof-
  from assms have A: "(x, y) \<in> A\<^sup>*" and B:"(x, y) \<notin> B\<^sup>*" by auto
  from A have "\<exists> k. (x, y) \<in> A^^k" by (rule rtrancl_imp_relpow)
  then obtain k where Ak:"(x, y) \<in> A^^k" by auto
  from Ak B show "(x, y) \<in> A\<^sup>* O (A - B) O A\<^sup>*"
  proof (induct k arbitrary: x)
    case 0
    with \<open>(x, y) \<notin> B\<^sup>*\<close> 0 show ?case using ccontr by auto
  next
    case (Suc i)
    then have B:"(x, y) \<notin> B\<^sup>*" and ASk:"(x, y) \<in> A ^^ Suc i" by auto
    from ASk have "\<exists>z. (x, z) \<in> A \<and> (z, y) \<in> A ^^ i" using relpow_Suc_D2 [where ?R=A] by auto
    then obtain z where xz:"(x, z) \<in> A" and "(z, y) \<in> A ^^ i" by auto
    then have zy:"(z, y) \<in> A\<^sup>*" using relpow_imp_rtrancl by auto
    from xz show "(x, y) \<in> A\<^sup>* O (A - B) O A\<^sup>*"
    proof (cases "(x, z) \<in> B")
      case False
      with xz zy show "(x, y) \<in> A\<^sup>* O (A - B) O A\<^sup>*" by auto
    next
      case True
      then have "(x, z) \<in> B\<^sup>*" by auto
      have "\<lbrakk>(x, z) \<in> B\<^sup>*; (z, y) \<in> B\<^sup>*\<rbrakk> \<Longrightarrow> (x, y) \<in> B\<^sup>*" using rtrancl_trans [of x z B] by auto
      with  \<open>(x, z) \<in> B\<^sup>*\<close> \<open>(x, y) \<notin> B\<^sup>*\<close> have "(z, y) \<notin> B\<^sup>*" by auto
      with Suc \<open>(z, y) \<in> A ^^ i\<close> have "(z, y) \<in> A\<^sup>* O (A - B) O A\<^sup>*" by auto
      with xz have xy:"(x, y) \<in> A O A\<^sup>* O (A - B) O A\<^sup>*" by auto
      have "A O A\<^sup>* O (A - B) O A\<^sup>* \<subseteq> A\<^sup>* O (A - B) O A\<^sup>*" by regexp
      from this xy show "(x, y) \<in> A\<^sup>* O (A - B) O A\<^sup>*"
        using subsetD [where ?A="A O A\<^sup>* O (A - B) O A\<^sup>*"] by auto
    qed
  qed
qed

lemma SN_empty [simp]: "SN {}" by auto

lemma SN_on_weakening:
  assumes "SN_on R1 A"
  shows "SN_on (R1 \<inter> R2) A"
proof -
  {
    assume "\<exists>S. S 0 \<in> A \<and> chain (R1 \<inter> R2) S"
    then obtain S where
      S0: "S 0 \<in> A" and
      SN: "chain (R1 \<inter> R2) S"
      by auto
    from SN have SN': "chain R1 S" by simp
    with S0 and assms have "False" by auto
  }
  then show ?thesis by force
qed

(* an explicit version of infinite reduction *)
definition ideriv :: "'a rel \<Rightarrow> 'a rel \<Rightarrow> (nat \<Rightarrow> 'a) \<Rightarrow> bool" where
  "ideriv R S as \<longleftrightarrow> (\<forall>i. (as i, as (Suc i)) \<in> R \<union> S) \<and> (INFM i. (as i, as (Suc i)) \<in> R)"

lemma ideriv_mono: "R \<subseteq> R' \<Longrightarrow> S \<subseteq> S' \<Longrightarrow> ideriv R S as \<Longrightarrow> ideriv R' S' as"
  unfolding ideriv_def INFM_nat by blast

fun
  shift :: "(nat \<Rightarrow> 'a) \<Rightarrow> nat \<Rightarrow> nat \<Rightarrow> 'a"
where
  "shift f j = (\<lambda> i. f (i+j))"

lemma ideriv_split:
  assumes ideriv: "ideriv R S as"
    and nideriv: "\<not> ideriv (D \<inter> (R \<union> S)) (R \<union> S - D) as"
  shows "\<exists> i. ideriv (R - D) (S - D) (shift as i)"
proof -
  have RS: "R - D \<union> (S - D) = R \<union> S - D" by auto
  from ideriv [unfolded ideriv_def]
  have as: "\<And> i. (as i, as (Suc i)) \<in> R \<union> S"
    and inf: "INFM i. (as i, as (Suc i)) \<in> R" by auto
  show ?thesis
  proof (cases "INFM i. (as i, as (Suc i)) \<in> D \<inter> (R \<union> S)")
    case True
    have "ideriv (D \<inter> (R \<union> S)) (R \<union> S - D) as"
      unfolding ideriv_def
      using as True  by auto
    with nideriv show ?thesis ..
  next
    case False
    from False [unfolded INFM_nat]
    obtain i where Dn: "\<And> j. i < j \<Longrightarrow> (as j, as (Suc j)) \<notin> D \<inter> (R \<union> S)"
      by auto
    from Dn as have as: "\<And> j. i < j \<Longrightarrow> (as j, as (Suc j)) \<in> R \<union> S - D" by auto
    show ?thesis
    proof (rule exI [of _ "Suc i"], unfold ideriv_def RS, insert as, intro conjI, simp, unfold INFM_nat, intro allI)
      fix m
      from inf [unfolded INFM_nat] obtain j where j: "j > Suc i + m"
        and R: "(as j, as (Suc j)) \<in> R" by auto
      with as [of j] have RD: "(as j, as (Suc j)) \<in> R - D" by auto      
      show "\<exists> j > m. (shift as (Suc i) j, shift as (Suc i) (Suc j)) \<in> R - D"
        by (rule exI [of _ "j - Suc i"], insert j RD, auto)
    qed
  qed
qed

lemma ideriv_SN:
  assumes SN: "SN S"
    and compat: "NS O S \<subseteq> S"
    and R: "R \<subseteq> NS \<union> S"
  shows "\<not> ideriv (S \<inter> R) (R - S) as"
proof
  assume "ideriv (S \<inter> R) (R - S) as"
  with R have steps: "\<forall> i. (as i, as (Suc i)) \<in> NS \<union> S"
    and inf: "INFM i. (as i, as (Suc i)) \<in> S \<inter> R" unfolding ideriv_def by auto
  from non_strict_ending [OF steps compat] SN
  obtain i where i: "\<And> j. j \<ge> i \<Longrightarrow> (as j, as (Suc j)) \<in> NS - S" by fast
  from inf [unfolded INFM_nat] obtain j where "j > i" and "(as j, as (Suc j)) \<in> S" by auto
  with i [of j] show False by auto
qed

lemma Infm_shift: "(INFM i. P (shift f n i)) = (INFM i. P (f i))" (is "?S = ?O")
proof 
  assume ?S
  show ?O
    unfolding INFM_nat_le 
  proof
    fix m
    from \<open>?S\<close> [unfolded INFM_nat_le]
    obtain k where k: "k \<ge> m" and p: "P (shift f n k)" by auto
    show "\<exists> k \<ge> m. P (f k)"
      by (rule exI [of _ "k + n"], insert k p, auto)
  qed
next
  assume ?O
  show ?S
    unfolding INFM_nat_le 
  proof
    fix m
    from \<open>?O\<close> [unfolded INFM_nat_le]
    obtain k where k: "k \<ge> m + n" and p: "P (f k)" by auto
    show "\<exists> k \<ge> m. P (shift f n k)"
      by (rule exI [of _ "k - n"], insert k p, auto)
  qed
qed

lemma rtrancl_list_conv:
  "(s, t) \<in> R\<^sup>* \<longleftrightarrow> 
    (\<exists> ts. last (s # ts) = t \<and> (\<forall>i<length ts. ((s # ts) ! i, (s # ts) ! Suc i) \<in> R))" (is "?l = ?r")
proof 
  assume ?r
  then obtain ts where "last (s # ts) = t \<and> (\<forall>i<length ts. ((s # ts) ! i, (s # ts) ! Suc i) \<in> R)" ..
  then show ?l
  proof (induct ts arbitrary: s, simp)
    case (Cons u ll)
    then have "last (u # ll) = t \<and> (\<forall>i<length ll. ((u # ll) ! i, (u # ll) ! Suc i) \<in> R)" by auto
    from Cons(1)[OF this] have rec: "(u, t) \<in> R\<^sup>*" .
    from Cons have "(s, u) \<in> R" by auto
    with rec show ?case by auto
  qed
next
  assume ?l
  from rtrancl_imp_seq [OF this]
  obtain S n where s: "S 0 = s" and t: "S n = t" and steps: "\<forall> i<n. (S i, S (Suc i)) \<in> R" by auto
  let ?ts = "map (\<lambda> i. S (Suc i)) [0 ..< n]"
  show ?r
  proof (rule exI [of _ ?ts], intro conjI, 
      cases n, simp add: s [symmetric] t [symmetric], simp add: t [symmetric]) 
    show "\<forall> i < length ?ts. ((s # ?ts) ! i, (s # ?ts) ! Suc i) \<in> R" 
    proof (intro allI impI)
      fix i
      assume i: "i < length ?ts"
      then show "((s # ?ts) ! i, (s # ?ts) ! Suc i) \<in> R"
      proof (cases i, simp add: s [symmetric] steps)
        case (Suc j)
        with i steps show ?thesis by simp
      qed
    qed
  qed
qed

lemma SN_reaches_NF:
  assumes "SN_on r {x}"
  shows "\<exists>y. (x, y) \<in> r\<^sup>* \<and> y \<in> NF r"
using assms
proof (induct rule: SN_on_induct')
  case (IH x)
  show ?case
  proof (cases "x \<in> NF r")
    case True
    then show ?thesis by auto
  next
    case False
    then obtain y where step: "(x, y) \<in> r" by auto
    from IH [OF this] obtain z where steps: "(y, z) \<in> r\<^sup>*" and NF: "z \<in> NF r" by auto
    show ?thesis
      by (intro exI, rule conjI [OF _ NF], insert step steps, auto)
  qed
qed

lemma SN_WCR_reaches_NF:
  assumes SN: "SN_on r {x}" 
    and WCR: "WCR_on r {x. SN_on r {x}}"
  shows "\<exists>! y. (x, y) \<in> r\<^sup>* \<and> y \<in> NF r"
proof -
  from SN_reaches_NF [OF SN] obtain y where steps: "(x, y) \<in> r\<^sup>*" and NF: "y \<in> NF r" by auto
  show ?thesis
  proof(rule, rule conjI [OF steps NF])
    fix z
    assume steps': "(x, z) \<in> r\<^sup>* \<and> z \<in> NF r"
    from Newman_local [OF SN WCR] have "CR_on r {x}" by auto
    from CR_onD [OF this _ steps] steps' have "(y, z) \<in> r\<^sup>\<down>" by simp
    from join_NF_imp_eq [OF this NF] steps' show "z = y" by simp
  qed
qed

definition some_NF :: "'a rel \<Rightarrow> 'a \<Rightarrow> 'a" where
  "some_NF r x = (SOME y. (x, y) \<in> r\<^sup>* \<and> y \<in> NF r)"

lemma some_NF:
  assumes SN: "SN_on r {x}" 
  shows "(x, some_NF r x) \<in> r\<^sup>* \<and> some_NF r x \<in> NF r"
  using someI_ex [OF SN_reaches_NF [OF SN]]
  unfolding some_NF_def .

lemma some_NF_WCR:
  assumes SN: "SN_on r {x}"
    and WCR: "WCR_on r {x. SN_on r {x}}"
    and steps: "(x, y) \<in> r\<^sup>*"
    and NF: "y \<in> NF r"
  shows "y = some_NF r x"
proof -
  let ?p = "\<lambda> y. (x, y) \<in> r\<^sup>* \<and> y \<in> NF r"
  from SN_WCR_reaches_NF [OF SN WCR]
  have one: "\<exists>! y. ?p y" .
  from steps NF have y: "?p y" ..
  from some_NF [OF SN] have some: "?p (some_NF r x)" .
  from one some y show ?thesis by auto
qed

lemma some_NF_UNF:
  assumes UNF: "UNF r"
    and steps: "(x, y) \<in> r\<^sup>*"
    and NF: "y \<in> NF r"
  shows "y = some_NF r x"
proof -
  let ?p = "\<lambda> y. (x, y) \<in> r\<^sup>* \<and> y \<in> NF r"
  from steps NF have py: "?p y" by simp
  then have pNF: "?p (some_NF r x)" unfolding some_NF_def 
    by (rule someI)
  from py have y: "(x, y) \<in> r\<^sup>!" by auto
  from pNF have nf: "(x, some_NF r x) \<in> r\<^sup>!" by auto
  from UNF [unfolded UNF_on_def] y nf show ?thesis by auto
qed

definition "the_NF A a = (THE b. (a, b) \<in> A\<^sup>!)"

context
  fixes A
  assumes SN: "SN A" and CR: "CR A"
begin
lemma the_NF: "(a, the_NF A a) \<in> A\<^sup>!"
proof -
  obtain b where ab: "(a, b) \<in> A\<^sup>!" using SN by (meson SN_imp_WN UNIV_I WN_onE)
  moreover have "(a, c) \<in> A\<^sup>! \<Longrightarrow> c = b" for c
    using CR and ab by (meson CR_divergence_imp_join join_NF_imp_eq normalizability_E)
  ultimately have "\<exists>!b. (a, b) \<in> A\<^sup>!" by blast
  then show ?thesis unfolding the_NF_def by (rule theI')
qed

lemma the_NF_NF: "the_NF A a \<in> NF A"
  using the_NF by (auto simp: normalizability_def)

lemma the_NF_step:
  assumes "(a, b) \<in> A"
  shows "the_NF A a = the_NF A b"
  using the_NF and assms
  by (meson CR SN SN_imp_WN conversionI' r_into_rtrancl semi_complete_imp_conversionIff_same_NF semi_complete_onI)

lemma the_NF_steps:
  assumes "(a, b) \<in> A\<^sup>*"
  shows "the_NF A a = the_NF A b"
  using assms by (induct) (auto dest: the_NF_step)

lemma the_NF_conv:
  assumes "(a, b) \<in> A\<^sup>\<leftrightarrow>\<^sup>*"
  shows "the_NF A a = the_NF A b"
  using assms
  by (meson CR WN_on_def the_NF semi_complete_imp_conversionIff_same_NF semi_complete_onI)

end


definition weak_diamond :: "'a rel \<Rightarrow> bool" ("w\<diamond>") where
  "w\<diamond> r \<longleftrightarrow> (r\<inverse> O r) - Id \<subseteq> (r O r\<inverse>)"

lemma weak_diamond_imp_CR:
  assumes wd: "w\<diamond> r"
  shows "CR r"
proof (rule semi_confluence_imp_CR, rule)
  fix x y
  assume "(x, y) \<in> r\<inverse> O r\<^sup>*"
  then obtain z where step: "(z, x) \<in> r" and steps: "(z, y) \<in> r\<^sup>*" by auto
  from steps
  have "\<exists> u. (x, u) \<in> r\<^sup>* \<and> (y, u) \<in> r\<^sup>=" 
  proof (induct) 
    case base
    show ?case
      by (rule exI [of _ x], insert step, auto)
  next
    case (step y' y)
    from step(3) obtain u where xu: "(x, u) \<in> r\<^sup>*" and y'u: "(y', u) \<in> r\<^sup>=" by auto
    from y'u have "(y', u) \<in> r \<or> y' = u" by auto
    then show ?case
    proof
      assume y'u: "y' = u"
      with xu step(2) have xy: "(x, y) \<in> r\<^sup>*" by auto
      show ?thesis 
        by (intro exI conjI, rule xy, simp)
    next
      assume "(y', u) \<in> r"
      with step(2) have uy: "(u, y) \<in> r\<inverse> O r" by auto
      show ?thesis
      proof (cases "u = y")
        case True
        show ?thesis
          by (intro exI conjI, rule xu, unfold True, simp)
      next
        case False
        with uy
          wd [unfolded weak_diamond_def] obtain u' where uu': "(u, u') \<in> r"
          and yu': "(y, u') \<in> r" by auto
        from xu uu' have xu: "(x, u') \<in> r\<^sup>*" by auto
        show ?thesis
          by (intro exI conjI, rule xu, insert yu', auto)
      qed
    qed
  qed        
  then show "(x, y) \<in> r\<^sup>\<down>" by auto
qed

lemma steps_imp_not_SN_on:
  fixes t :: "'a \<Rightarrow> 'b"
    and R :: "'b rel"
  assumes steps: "\<And> x. (t x, t (f x)) \<in> R"
  shows "\<not> SN_on R {t x}"
proof  
  let ?U = "range t"
  assume "SN_on R {t x}"
  from SN_on_imp_on_minimal [OF this, rule_format, of ?U]
  obtain tz where tz: "tz \<in> range t" and min: "\<And> y. (tz, y) \<in> R \<Longrightarrow> y \<notin> range t" by auto
  from tz obtain z where tz: "tz = t z" by auto
  from steps [of z] min [of "t (f z)"] show False unfolding tz by auto
qed

lemma steps_imp_not_SN:
  fixes t :: "'a \<Rightarrow> 'b"
    and R :: "'b rel"
  assumes steps: "\<And> x. (t x, t (f x)) \<in> R"
  shows "\<not> SN R"
proof -
  from steps_imp_not_SN_on [of t f R, OF steps]
  show ?thesis unfolding SN_def by blast
qed

lemma steps_map:
  assumes fg: "\<And>t u R . P t \<Longrightarrow> Q R \<Longrightarrow> (t, u) \<in> R \<Longrightarrow> P u \<and> (f t, f u) \<in> g R"
  and t: "P t"
  and R: "Q R"
  and S: "Q S"
  shows "((t, u) \<in> R\<^sup>* \<longrightarrow> (f t, f u) \<in> (g R)\<^sup>*)
    \<and> ((t, u) \<in> R\<^sup>* O S O R\<^sup>* \<longrightarrow> (f t, f u) \<in> (g R)\<^sup>* O (g S) O (g R)\<^sup>*)"
proof -
  {
    fix t u
    assume "(t, u) \<in> R\<^sup>*" and "P t"
    then have "P u \<and> (f t, f u) \<in> (g R)\<^sup>*"
    proof (induct)
      case (step u v)
      from step(3)[OF step(4)] have Pu: "P u" and steps: "(f t, f u) \<in> (g R)\<^sup>*" by auto
      from fg [OF Pu R step(2)] have Pv: "P v" and step: "(f u, f v) \<in> g R" by auto
      with steps have "(f t, f v) \<in> (g R)\<^sup>*" by auto
      with Pv show ?case by simp
    qed simp
  } note main = this
  note maint = main [OF _ t]
  from maint [of u] have one: "(t, u) \<in> R\<^sup>* \<longrightarrow> (f t, f u) \<in> (g R)\<^sup>*" by simp
  show ?thesis
  proof (rule conjI [OF one impI])
    assume "(t, u) \<in> R\<^sup>* O S O R\<^sup>*"
    then obtain s v where ts: "(t, s) \<in> R\<^sup>*" and sv: "(s, v) \<in> S" and vu: "(v, u) \<in> R\<^sup>*" by auto
    from maint [OF ts] have Ps: "P s" and ts: "(f t, f s) \<in> (g R)\<^sup>*" by auto
    from fg [OF Ps S sv] have Pv: "P v" and sv: "(f s, f v) \<in> g S" by auto
    from main [OF vu Pv] have vu: "(f v, f u) \<in> (g R)\<^sup>*" by auto
    from ts sv vu show "(f t, f u) \<in> (g R)\<^sup>* O g S O (g R)\<^sup>*" by auto
  qed
qed


subsection \<open>Terminating part of a relation\<close>

inductive_set
  SN_part :: "'a rel \<Rightarrow> 'a set"
  for r :: "'a rel"
where
  SN_partI: "(\<And>y. (x, y) \<in> r \<Longrightarrow> y \<in> SN_part r) \<Longrightarrow> x \<in> SN_part r"

text \<open>The accessible part of a relation is the same as the terminating part
(just two names for the same definition -- modulo argument order). See
@{thm accI}.\<close>

text \<open>Characterization of @{const SN_on} via terminating part.\<close>
lemma SN_on_SN_part_conv:
  "SN_on r A \<longleftrightarrow> A \<subseteq> SN_part r"
proof -
  {
    fix x assume "SN_on r A" and "x \<in> A"
    then have "x \<in> SN_part r" by (induct) (auto intro: SN_partI)
  } moreover {
    fix x assume "x \<in> A" and "A \<subseteq> SN_part r"
    then have "x \<in> SN_part r" by auto
    then have "SN_on r {x}" by (induct) (auto intro: step_reflects_SN_on)
  } ultimately show ?thesis by (force simp: SN_defs)
qed

text \<open>Special case for ``full'' termination.\<close>
lemma SN_SN_part_UNIV_conv:
  "SN r \<longleftrightarrow> SN_part r = UNIV"
  using SN_on_SN_part_conv [of r UNIV] by auto

lemma closed_imp_rtrancl_closed: assumes L: "L \<subseteq> A"
  and R: "R `` A \<subseteq> A"
  shows "{t | s. s \<in> L \<and> (s,t) \<in> R^*} \<subseteq> A"
proof -
  {
    fix s t
    assume "(s,t) \<in> R^*" and "s \<in> L" 
    hence "t \<in> A"
      by (induct, insert L R, auto)
  }
  thus ?thesis by auto
qed

lemma trancl_steps_relpow: assumes "a \<subseteq> b^+"
  shows "(x,y) \<in> a^^n \<Longrightarrow> \<exists> m. m \<ge> n \<and> (x,y) \<in> b^^m"
proof (induct n arbitrary: y)
  case 0 thus ?case by (intro exI[of _ 0], auto)
next
  case (Suc n z)
  from Suc(2) obtain y where xy: "(x,y) \<in> a ^^ n" and yz: "(y,z) \<in> a" by auto
  from Suc(1)[OF xy] obtain m where m: "m \<ge> n" and xy: "(x,y) \<in> b ^^ m" by auto
  from yz assms have "(y,z) \<in> b^+" by auto
  from this[unfolded trancl_power] obtain k where k: "k > 0" and yz: "(y,z) \<in> b ^^ k" by auto
  from xy yz have "(x,z) \<in> b ^^ (m + k)" unfolding relpow_add by auto
  with k m show ?case by (intro exI[of _ "m + k"], auto)
qed

lemma relpow_image: assumes f: "\<And> s t. (s,t) \<in> r \<Longrightarrow> (f s, f t) \<in> r'"
  shows "(s,t) \<in> r ^^ n \<Longrightarrow> (f s, f t) \<in> r' ^^ n"
proof (induct n arbitrary: t)
  case (Suc n u)
  from Suc(2) obtain t where st: "(s,t) \<in> r ^^ n" and tu: "(t,u) \<in> r" by auto
  from Suc(1)[OF st] f[OF tu] show ?case by auto
qed auto

lemma relpow_refl_mono:
 assumes refl:"\<And> x. (x,x) \<in> Rel"
 shows "m \<le> n \<Longrightarrow>(a,b) \<in> Rel ^^ m \<Longrightarrow> (a,b) \<in> Rel ^^ n"
proof (induct rule:dec_induct)
  case (step i)
  hence abi:"(a, b) \<in> Rel ^^ i" by auto
  from refl[of b] abi relpowp_Suc_I[of i "\<lambda> x y. (x,y) \<in> Rel"] show "(a, b) \<in> Rel ^^ Suc i" by auto
qed

lemma SN_on_induct_acc_style [consumes 1, case_names IH]:
  assumes sn: "SN_on R {a}"
    and IH: "\<And>x. SN_on R {x} \<Longrightarrow> \<lbrakk>\<And>y. (x, y) \<in> R \<Longrightarrow> P y\<rbrakk>  \<Longrightarrow> P x"
  shows "P a"
proof -
  from sn SN_on_conv_acc [of "R\<inverse>" a] have a: "a \<in> termi R" by auto
  show ?thesis
  proof (rule Wellfounded.acc.induct [OF a, of P], rule IH)
    fix x
    assume "\<And>y. (y, x) \<in> R\<inverse> \<Longrightarrow> y \<in> termi R"
    from this [folded SN_on_conv_acc]
      show "SN_on R {x}" by simp fast
  qed auto
qed

(* Lemma 2.3 in Huet: Confluent Reductions *)
lemma partially_localize_CR:
  "CR r \<longleftrightarrow> (\<forall> x y z. (x, y) \<in> r \<and> (x, z) \<in> r\<^sup>* \<longrightarrow> (y, z) \<in> join r)" 
proof
  assume "CR r"
  thus "\<forall> x y z. (x, y) \<in> r \<and> (x, z) \<in> r\<^sup>* \<longrightarrow> (y, z) \<in> join r" by auto
next
  assume 1:"\<forall> x y z. (x, y) \<in> r \<and> (x, z) \<in> r\<^sup>* \<longrightarrow> (y, z) \<in> join r" 
  show "CR r"
  proof
    fix a b c
    assume 2: "a \<in> UNIV" and 3: "(a, b) \<in> r\<^sup>*" and 4: "(a, c) \<in> r\<^sup>*"
    then obtain n  where "(a,c) \<in> r^^n" using rtrancl_is_UN_relpow by fast
    with 2 3 show "(b,c) \<in> join r" 
    proof (induct n arbitrary: a b c)
      case 0 thus ?case by auto
    next
      case (Suc m)
      from Suc(4) obtain d where ad: "(a, d) \<in> r^^m" and dc: "(d, c) \<in> r" by auto
      from Suc(1) [OF Suc(2) Suc(3) ad] have "(b, d) \<in> join r" .
      with 1 dc joinE joinI [of b _ r c] join_rtrancl_join show ?case by metis
    qed
  qed
qed

definition strongly_confluent_on :: "'a rel \<Rightarrow> 'a set \<Rightarrow> bool"
where 
  "strongly_confluent_on r A \<longleftrightarrow>
    (\<forall>x \<in> A.  \<forall>y z. (x, y) \<in> r \<and> (x, z) \<in> r \<longrightarrow> (\<exists>u. (y, u) \<in> r\<^sup>* \<and> (z, u) \<in> r\<^sup>=))"

abbreviation strongly_confluent :: "'a rel \<Rightarrow> bool"
where
  "strongly_confluent r \<equiv> strongly_confluent_on r UNIV"

lemma strongly_confluent_on_E11:
  "strongly_confluent_on r A \<Longrightarrow> x \<in> A \<Longrightarrow> (x, y) \<in> r \<Longrightarrow> (x, z) \<in> r \<Longrightarrow>
    \<exists>u. (y, u) \<in> r\<^sup>* \<and> (z, u) \<in> r\<^sup>="
unfolding strongly_confluent_on_def by blast

lemma strongly_confluentI [intro]:
  "\<lbrakk>\<And>x y z. (x, y) \<in> r \<Longrightarrow> (x, z) \<in> r \<Longrightarrow> \<exists>u. (y, u) \<in> r\<^sup>* \<and> (z, u) \<in> r\<^sup>=\<rbrakk> \<Longrightarrow> strongly_confluent r" 
unfolding strongly_confluent_on_def by auto

lemma strongly_confluent_E1n:
  assumes scr: "strongly_confluent r"
  shows "(x, y) \<in> r\<^sup>= \<Longrightarrow> (x, z) \<in> r ^^ n \<Longrightarrow> \<exists>u. (y, u) \<in> r\<^sup>* \<and> (z, u) \<in> r\<^sup>="
proof (induct n arbitrary: x y z)
  case (Suc m)
  from Suc(3) obtain w where xw: "(x, w) \<in> r^^m" and wz: "(w, z) \<in> r" by auto
  from Suc(1) [OF Suc(2) xw] obtain u where yu: "(y, u) \<in> r\<^sup>*" and wu: "(w, u) \<in> r\<^sup>=" by auto
  from strongly_confluent_on_E11 [OF scr, of w] wz yu wu show ?case 
    by (metis UnE converse_rtrancl_into_rtrancl iso_tuple_UNIV_I pair_in_Id_conv rtrancl_trans)
qed auto

(* Lemma 2.5 in Huet: Confluent Reductions *)
lemma strong_confluence_imp_CR: 
  assumes "strongly_confluent r"
  shows "CR r"
proof -
  { fix x y z
    have "(x, y) \<in> r \<Longrightarrow> (x, z) \<in> r\<^sup>* \<Longrightarrow> (y, z) \<in> join r" 
      by (cases "x = y", insert strongly_confluent_E1n [OF assms], blast+) }
  then show "CR r" using partially_localize_CR by blast
qed

lemma WCR_alt_def: "WCR A \<longleftrightarrow> A\<inverse> O A \<subseteq> A\<^sup>\<down>" by (auto simp: WCR_defs)

lemma NF_imp_SN_on: "a \<in> NF R \<Longrightarrow> SN_on R {a}" unfolding SN_on_def NF_def by blast

lemma Union_sym: "(s, t) \<in> (\<Union>i\<le>n. (S i)\<^sup>\<leftrightarrow>) \<longleftrightarrow> (t, s) \<in> (\<Union>i\<le>n. (S i)\<^sup>\<leftrightarrow>)" by auto

lemma peak_iff: "(x, y) \<in> A\<inverse> O B \<longleftrightarrow> (\<exists>u. (u, x) \<in> A \<and> (u, y) \<in> B)" by auto

lemma CR_NF_conv:
  assumes "CR r" and "t \<in> NF r" and "(u, t) \<in> r\<^sup>\<leftrightarrow>\<^sup>*"
  shows "(u, t) \<in> r\<^sup>!"
using assms
unfolding CR_imp_conversionIff_join [OF \<open>CR r\<close>]
by (auto simp: NF_iff_no_step normalizability_def)
   (metis (mono_tags) converse_rtranclE joinE)

lemma NF_join_imp_reach:
  assumes "y \<in> NF A" and "(x, y) \<in> A\<^sup>\<down>"
  shows "(x, y) \<in> A\<^sup>*"
using assms by (auto simp: join_def) (metis NF_not_suc rtrancl_converseD)

lemma conversion_O_conversion [simp]:
  "A\<^sup>\<leftrightarrow>\<^sup>* O A\<^sup>\<leftrightarrow>\<^sup>* = A\<^sup>\<leftrightarrow>\<^sup>*"
  by (force simp: converse_def)

lemma trans_O_iff: "trans A \<longleftrightarrow> A O A \<subseteq> A" unfolding trans_def by auto
lemma refl_O_iff: "refl A \<longleftrightarrow> Id \<subseteq> A" unfolding refl_on_def by auto

lemma relpow_Suc: "r ^^ Suc n = r O r ^^ n"
  using relpow_add[of 1 n r] by auto

lemma converse_power: fixes r :: "'a rel" shows "(r\<inverse>)^^n = (r^^n)\<inverse>"
proof (induct n)
  case (Suc n)
  show ?case unfolding relpow.simps(2)[of _ "r\<inverse>"] relpow_Suc[of _ r]
    by (simp add: Suc converse_relcomp)
qed simp

lemma conversion_mono: "A \<subseteq> B \<Longrightarrow> A\<^sup>\<leftrightarrow>\<^sup>* \<subseteq> B\<^sup>\<leftrightarrow>\<^sup>*"
by (auto simp: conversion_def intro!: rtrancl_mono)

lemma conversion_conversion_idemp [simp]: "(A\<^sup>\<leftrightarrow>\<^sup>*)\<^sup>\<leftrightarrow>\<^sup>* = A\<^sup>\<leftrightarrow>\<^sup>*"
  by auto

lemma lower_set_imp_not_SN_on:
  assumes "s \<in> X" "\<forall>t \<in> X. \<exists>u \<in> X. (t,u) \<in> R" shows "\<not> SN_on R {s}"
  by (meson SN_on_imp_on_minimal assms)


lemma SN_on_Image_rtrancl_iff[simp]: "SN_on R (R\<^sup>* `` X) \<longleftrightarrow> SN_on R X" (is "?l = ?r")
proof(intro iffI)
  assume "?l" show "?r" by (rule SN_on_subset2[OF _ \<open>?l\<close>], auto)
qed (fact SN_on_Image_rtrancl)

lemma O_mono1: "R \<subseteq> R' \<Longrightarrow> S O R \<subseteq> S O R'" by auto
lemma O_mono2: "R \<subseteq> R' \<Longrightarrow> R O T \<subseteq> R' O T" by auto

lemma rtrancl_O_shift: "(S O R)\<^sup>* O S = S O (R O S)\<^sup>*"
  (* regexp does not work, since R is of type 'a x 'b set, not 'a rel *)
proof(intro equalityI subrelI)
  fix x y
  assume "(x,y) \<in> (S O R)\<^sup>* O S"
  then obtain n where "(x,y) \<in> (S O R)^^n O S" by blast
  then show "(x,y) \<in> S O (R O S)\<^sup>*"
  proof(induct n arbitrary: y)
    case IH: (Suc n)
    then obtain z where xz: "(x,z) \<in> (S O R)^^n O S" and zy: "(z,y) \<in> R O S" by auto
    from IH.hyps[OF xz] zy have "(x,y) \<in> S O (R O S)\<^sup>* O R O S" by auto
    then show ?case by(fold trancl_unfold_right, auto)
  qed auto
next
  fix x y
  assume "(x,y) \<in> S O (R O S)\<^sup>*"
  then obtain n where "(x,y) \<in> S O (R O S)^^n" by blast
  then show "(x,y) \<in> (S O R)\<^sup>* O S"
  proof(induct n arbitrary: y)
    case IH: (Suc n)
    then obtain z where xz: "(x,z) \<in> S O (R O S)^^n" and zy: "(z,y) \<in> R O S" by auto
    from IH.hyps[OF xz] zy have "(x,y) \<in> ((S O R)\<^sup>* O S O R) O S" by auto
    from this[folded trancl_unfold_right]
    show ?case by (rule rev_subsetD[OF _ O_mono2], auto simp: O_assoc)
  qed auto
qed

lemma O_rtrancl_O_O: "R O (S O R)\<^sup>* O S = (R O S)\<^sup>+"
  by (unfold rtrancl_O_shift trancl_unfold_left, auto)

lemma SN_on_subset_SN_terms:
  assumes SN: "SN_on R X" shows "X \<subseteq> {x. SN_on R {x}}"
proof(intro subsetI, unfold mem_Collect_eq)
  fix x assume x: "x \<in> X"
  show "SN_on R {x}" by (rule SN_on_subset2[OF _ SN], insert x, auto)
qed

lemma SN_on_Un2:
  assumes "SN_on R X" and "SN_on R Y" shows "SN_on R (X \<union> Y)"
  using assms by fast

lemma SN_on_UN:
  assumes "\<And>x. SN_on R (X x)" shows "SN_on R (\<Union>x. X x)"
  using assms by fast

lemma Image_subsetI: "R \<subseteq> R' \<Longrightarrow> R `` X \<subseteq> R' `` X" by auto

lemma SN_on_O_comm:
  assumes SN: "SN_on ((R :: ('a\<times>'b) set) O (S :: ('b\<times>'a) set)) (S `` X)"
  shows "SN_on (S O R) X"
proof
  fix seq :: "nat \<Rightarrow> 'b" assume seq0: "seq 0 \<in> X" and chain: "chain (S O R) seq"
  from SN have SN: "SN_on (R O S) ((R O S)\<^sup>* `` S `` X)" by simp
  { fix i a
    assume ia: "(seq i,a) \<in> S" and aSi: "(a,seq (Suc i)) \<in> R"
    have "seq i \<in> (S O R)\<^sup>* `` X"
    proof (induct i)
      case 0 from seq0 show ?case by auto
    next
      case (Suc i) with chain have "seq (Suc i) \<in> ((S O R)\<^sup>* O S O R) `` X" by blast
      also have "... \<subseteq> (S O R)\<^sup>* `` X" by (fold trancl_unfold_right, auto)
      finally show ?case.
    qed
    with ia have "a \<in> ((S O R)\<^sup>* O S) `` X" by auto
    then have a: "a \<in> ((R O S)\<^sup>*) `` S `` X" by (auto simp: rtrancl_O_shift)
    with ia aSi have False
    proof(induct "a" arbitrary: i rule: SN_on_induct[OF SN])
      case 1 show ?case by (fact a)
    next
      case IH: (2 a)
      from chain obtain b
      where *: "(seq (Suc i), b) \<in> S" "(b, seq (Suc (Suc i))) \<in> R" by auto
      with IH have ab: "(a,b) \<in> R O S" by auto
      with \<open>a \<in> (R O S)\<^sup>* `` S `` X\<close> have "b \<in> ((R O S)\<^sup>* O R O S) `` S `` X" by auto
      then have "b \<in> (R O S)\<^sup>* `` S `` X"
        by (rule rev_subsetD, intro Image_subsetI, fold trancl_unfold_right, auto)
      from IH.hyps[OF ab * this] IH.prems ab show False by auto
    qed
  }
  with chain show False by auto
qed

lemma SN_O_comm: "SN (R O S) \<longleftrightarrow> SN (S O R)"
  by (intro iffI; rule SN_on_O_comm[OF SN_on_subset2], auto)

lemma chain_mono: assumes "R' \<subseteq> R" "chain R' seq" shows "chain R seq"
  using assms by auto

context
  fixes S R
  assumes push: "S O R \<subseteq> R O S\<^sup>*"
begin

lemma rtrancl_O_push: "S\<^sup>* O R \<subseteq> R O S\<^sup>*"
proof-
  { fix n
    have "\<And>s t. (s,t) \<in> S ^^ n O R \<Longrightarrow> (s,t) \<in> R O S\<^sup>*"
    proof(induct n)
      case (Suc n)
        then obtain u where "(s,u) \<in> S" "(u,t) \<in> R O S\<^sup>*" unfolding relpow_Suc by blast
        then have "(s,t) \<in> S O R O S\<^sup>*" by auto
        also have "... \<subseteq> R O S\<^sup>* O S\<^sup>*" using push by blast
        also have "... \<subseteq> R O S\<^sup>*" by auto
        finally show ?case.
    qed auto
  }
  thus ?thesis by blast
qed

lemma rtrancl_U_push: "(S \<union> R)\<^sup>* = R\<^sup>* O S\<^sup>*"
proof(intro equalityI subrelI)
  fix x y
  assume "(x,y) \<in> (S \<union> R)\<^sup>*"
  also have "... \<subseteq> (S\<^sup>* O R)\<^sup>* O S\<^sup>*" by regexp
  finally obtain z where xz: "(x,z) \<in> (S\<^sup>* O R)\<^sup>*" and zy: "(z,y) \<in> S\<^sup>*" by auto
  from xz have "(x,z) \<in> R\<^sup>* O S\<^sup>*"
  proof (induct rule: rtrancl_induct)
    case (step z w)
      then have "(x,w) \<in> R\<^sup>* O S\<^sup>* O S\<^sup>* O R" by auto
      also have "... \<subseteq> R\<^sup>* O S\<^sup>* O R" by regexp
      also have "... \<subseteq> R\<^sup>* O R O S\<^sup>*" using rtrancl_O_push by auto
      also have "... \<subseteq> R\<^sup>* O S\<^sup>*" by regexp
      finally show ?case.
  qed auto
  with zy show "(x,y) \<in> R\<^sup>* O S\<^sup>*" by auto
qed regexp

lemma SN_on_O_push:
  assumes SN: "SN_on R X" shows "SN_on (R O S\<^sup>*) X"
proof
  fix seq
  have SN: "SN_on R (R\<^sup>* `` X)" using SN_on_Image_rtrancl[OF SN].
  moreover assume "seq (0::nat) \<in> X"
    then have "seq 0 \<in> R\<^sup>* `` X" by auto
  ultimately show "chain (R O S\<^sup>*) seq \<Longrightarrow> False"
  proof(induct "seq 0" arbitrary: seq rule: SN_on_induct)
    case IH
    then have 01: "(seq 0, seq 1) \<in> R O S\<^sup>*"
          and 12: "(seq 1, seq 2) \<in> R O S\<^sup>*"
          and 23: "(seq 2, seq 3) \<in> R O S\<^sup>*" by (auto simp: eval_nat_numeral)
    then obtain s t
    where s: "(seq 0, s) \<in> R" and s1: "(s, seq 1) \<in> S\<^sup>*"
      and t: "(seq 1, t) \<in> R" and t2: "(t, seq 2) \<in> S\<^sup>*" by auto
    from s1 t have "(s,t) \<in> S\<^sup>* O R" by auto
    with rtrancl_O_push have st: "(s,t) \<in> R O S\<^sup>*" by auto
    from t2 23 have "(t, seq 3) \<in> S\<^sup>* O R O S\<^sup>*" by auto
    also from rtrancl_O_push have "... \<subseteq> R O S\<^sup>* O S\<^sup>*" by blast
    finally have t3: "(t, seq 3) \<in> R O S\<^sup>*" by regexp
    let ?seq = "\<lambda>i. case i of 0 \<Rightarrow> s | Suc 0 \<Rightarrow> t | i \<Rightarrow> seq (Suc i)"
    show ?case
    proof(rule IH)
      from s show "(seq 0, ?seq 0) \<in> R" by auto
      show "chain (R O S\<^sup>*) ?seq"
      proof (intro allI)
        fix i show "(?seq i, ?seq (Suc i)) \<in> R O S\<^sup>*"
        proof (cases i)
          case 0 with st show ?thesis by auto
        next
          case (Suc i) with t3 IH show ?thesis by (cases i, auto simp: eval_nat_numeral)
        qed
      qed
    qed
  qed
qed

lemma SN_on_Image_push:
  assumes SN: "SN_on R X" shows "SN_on R (S\<^sup>* `` X)"
proof-
  { fix n
    have "SN_on R ((S^^n) `` X)"
    proof(induct n)
      case 0 from SN show ?case by auto
      case (Suc n)
        from SN_on_O_push[OF this] have "SN_on (R O S\<^sup>*) ((S ^^ n) `` X)".
        from SN_on_Image[OF this]
        have "SN_on (R O S\<^sup>*) ((R O S\<^sup>*) `` (S ^^ n) `` X)".
        then have "SN_on R ((R O S\<^sup>*) `` (S ^^ n) `` X)" by (rule SN_on_mono, auto)
        from SN_on_subset2[OF Image_mono[OF push subset_refl] this]
        have "SN_on R (R `` (S ^^ Suc n) `` X)" by (auto simp: relcomp_Image)
        then show ?case by fast
    qed
  }
  then show ?thesis by fast
qed

end

lemma not_SN_onI[intro]: "f 0 \<in> X \<Longrightarrow> chain R f \<Longrightarrow> \<not> SN_on R X"
  by (unfold SN_on_def not_not, intro exI conjI)
lemma shift_comp[simp]: "shift (f \<circ> seq) n = f \<circ> (shift seq n)" by auto

lemma Id_on_union: "Id_on (A \<union> B) = Id_on A \<union> Id_on B" unfolding Id_on_def by auto

lemma relpow_union_cases: "(a,d) \<in> (A \<union> B)^^n \<Longrightarrow> (a,d) \<in> B^^n \<or> (\<exists> b c k m. (a,b) \<in> B^^k \<and> (b,c) \<in> A \<and> (c,d) \<in> (A \<union> B)^^m \<and> n = Suc (k + m))"
proof (induct n arbitrary: a d)
  case (Suc n a e)
  let ?AB = "A \<union> B"
  from Suc(2) obtain b where ab: "(a,b) \<in> ?AB" and be: "(b,e) \<in> ?AB^^n" by (rule relpow_Suc_E2)
  from ab
  show ?case
  proof
    assume "(a,b) \<in> A"
    show ?thesis
    proof (rule disjI2, intro exI conjI)
      show "Suc n = Suc (0 + n)" by simp
      show "(a,b) \<in> A" by fact
    qed (insert be, auto)
  next
    assume ab: "(a,b) \<in> B"
    from Suc(1)[OF be]
    show ?thesis
    proof
      assume "(b,e) \<in> B ^^ n"
      with ab show ?thesis
        by (intro disjI1 relpow_Suc_I2)
    next
      assume "\<exists> c d k m. (b, c) \<in> B ^^ k \<and> (c, d) \<in> A \<and> (d, e) \<in> ?AB ^^ m \<and> n = Suc (k + m)"
      then obtain c d k m where "(b, c) \<in> B ^^ k" and *: "(c, d) \<in> A" "(d, e) \<in> ?AB ^^ m" "n = Suc (k + m)" by blast
      with ab have ac: "(a,c) \<in> B ^^ (Suc k)" by (intro relpow_Suc_I2)
      show ?thesis
        by (intro disjI2 exI conjI, rule ac, (rule *)+, simp add: *)
    qed
  qed
qed simp

lemma trans_refl_imp_rtrancl_id:
  assumes "trans r" "refl r"
  shows "r\<^sup>* = r"
proof
  show "r\<^sup>* \<subseteq> r"
  proof
    fix x y
    assume "(x,y) \<in> r\<^sup>*"
    thus "(x,y) \<in> r"
      by (induct, insert assms, unfold refl_on_def trans_def, blast+)
  qed
qed regexp

lemma trans_refl_imp_O_id:
  assumes "trans r" "refl r"
  shows "r O r = r"
proof(intro equalityI)
  show "r O r \<subseteq> r" by(fact trans_O_subset[OF assms(1)])
  have "r \<subseteq> r O Id" by auto
  moreover have "Id \<subseteq> r" by(fact assms(2)[unfolded refl_O_iff])
  ultimately show "r \<subseteq> r O r" by auto
qed

lemma relcomp3_I:
  assumes "(t, u) \<in> A" and "(s, t) \<in> B" and "(u, v) \<in> B"
  shows "(s, v) \<in> B O A O B"
  using assms by blast

lemma relcomp3_transI:
  assumes "trans B" and "(t, u) \<in> B O A O B" and "(s, t) \<in> B" and "(u, v) \<in> B"
  shows "(s, v) \<in> B O A O B"
using assms by (auto simp: trans_def intro: relcomp3_I)

lemmas converse_inward = rtrancl_converse[symmetric] converse_Un converse_UNION converse_relcomp
  converse_converse converse_Id

lemma qc_SN_relto_iff:
  assumes "r O s \<subseteq> s O (s \<union> r)\<^sup>*"
  shows "SN (r\<^sup>* O s O r\<^sup>*) = SN s"
proof -
  from converse_mono [THEN iffD2 , OF assms]
  have *: "s\<inverse> O r\<inverse> \<subseteq> (s\<inverse> \<union> r\<inverse>)\<^sup>* O s\<inverse>" unfolding converse_inward .
  have "(r\<^sup>* O s O r\<^sup>*)\<inverse> = (r\<inverse>)\<^sup>* O s\<inverse> O (r\<inverse>)\<^sup>*"
    by (simp only: converse_relcomp O_assoc rtrancl_converse)
  with qc_wf_relto_iff [OF *]
  show ?thesis by (simp add: SN_iff_wf)
qed

lemma conversion_empty [simp]: "conversion {} = Id"
  by (auto simp: conversion_def)

lemma symcl_idemp [simp]: "(r\<^sup>\<leftrightarrow>)\<^sup>\<leftrightarrow> = r\<^sup>\<leftrightarrow>" by auto

end