Theory Zorn

(*  Title:      HOL/Library/Zorn.thy
    Author:     Jacques D. Fleuriot, Tobias Nipkow

Zorn's Lemma (ported from Larry Paulson's Zorn.thy in ZF).
The well-ordering theorem.
*)

header {* Zorn's Lemma *}

theory Zorn
imports Order_Relation
begin

(* Define globally? In Set.thy? *)
definition chain_subset :: "'a set set => bool" ("chain⇘⊆⇙")
where
  "chain⇘⊆⇙ C ≡ ∀A∈C.∀B∈C. A ⊆ B ∨ B ⊆ A"

text{*
  The lemma and section numbers refer to an unpublished article
  \cite{Abrial-Laffitte}.
*}

definition chain :: "'a set set => 'a set set set"
where
  "chain S = {F. F ⊆ S ∧ chain⇘⊆⇙ F}"

definition super :: "'a set set => 'a set set => 'a set set set"
where
  "super S c = {d. d ∈ chain S ∧ c ⊂ d}"

definition maxchain  ::  "'a set set => 'a set set set"
where
  "maxchain S = {c. c ∈ chain S ∧ super S c = {}}"

definition succ :: "'a set set => 'a set set => 'a set set"
where
  "succ S c = (if c ∉ chain S ∨ c ∈ maxchain S then c else SOME c'. c' ∈ super S c)"

inductive_set TFin :: "'a set set => 'a set set set"
for S :: "'a set set"
where
  succI:      "x ∈ TFin S ==> succ S x ∈ TFin S"
| Pow_UnionI: "Y ∈ Pow (TFin S) ==> \<Union>Y ∈ TFin S"


subsection{*Mathematical Preamble*}

lemma Union_lemma0:
    "(∀x ∈ C. x ⊆ A | B ⊆ x) ==> Union(C) ⊆ A | B ⊆ Union(C)"
  by blast


text{*This is theorem @{text increasingD2} of ZF/Zorn.thy*}

lemma Abrial_axiom1: "x ⊆ succ S x"
  apply (auto simp add: succ_def super_def maxchain_def)
  apply (rule contrapos_np, assumption)
  apply (rule someI2)
  apply blast+
  done

lemmas TFin_UnionI = TFin.Pow_UnionI [OF PowI]

lemma TFin_induct:
  assumes H: "n ∈ TFin S" and
    I: "!!x. x ∈ TFin S ==> P x ==> P (succ S x)"
      "!!Y. Y ⊆ TFin S ==> Ball Y P ==> P (Union Y)"
  shows "P n"
  using H by induct (blast intro: I)+

lemma succ_trans: "x ⊆ y ==> x ⊆ succ S y"
  apply (erule subset_trans)
  apply (rule Abrial_axiom1)
  done

text{*Lemma 1 of section 3.1*}
lemma TFin_linear_lemma1:
     "[| n ∈ TFin S;  m ∈ TFin S;
         ∀x ∈ TFin S. x ⊆ m --> x = m | succ S x ⊆ m
      |] ==> n ⊆ m | succ S m ⊆ n"
  apply (erule TFin_induct)
   apply (erule_tac [2] Union_lemma0)
  apply (blast del: subsetI intro: succ_trans)
  done

text{* Lemma 2 of section 3.2 *}
lemma TFin_linear_lemma2:
     "m ∈ TFin S ==> ∀n ∈ TFin S. n ⊆ m --> n=m | succ S n ⊆ m"
  apply (erule TFin_induct)
   apply (rule impI [THEN ballI])
   txt{*case split using @{text TFin_linear_lemma1}*}
   apply (rule_tac n1 = n and m1 = x in TFin_linear_lemma1 [THEN disjE],
     assumption+)
    apply (drule_tac x = n in bspec, assumption)
    apply (blast del: subsetI intro: succ_trans, blast)
  txt{*second induction step*}
  apply (rule impI [THEN ballI])
  apply (rule Union_lemma0 [THEN disjE])
    apply (rule_tac [3] disjI2)
    prefer 2 apply blast
   apply (rule ballI)
   apply (rule_tac n1 = n and m1 = x in TFin_linear_lemma1 [THEN disjE],
     assumption+, auto)
  apply (blast intro!: Abrial_axiom1 [THEN subsetD])
  done

text{*Re-ordering the premises of Lemma 2*}
lemma TFin_subsetD:
     "[| n ⊆ m;  m ∈ TFin S;  n ∈ TFin S |] ==> n=m | succ S n ⊆ m"
  by (rule TFin_linear_lemma2 [rule_format])

text{*Consequences from section 3.3 -- Property 3.2, the ordering is total*}
lemma TFin_subset_linear: "[| m ∈ TFin S;  n ∈ TFin S|] ==> n ⊆ m | m ⊆ n"
  apply (rule disjE)
    apply (rule TFin_linear_lemma1 [OF _ _TFin_linear_lemma2])
      apply (assumption+, erule disjI2)
  apply (blast del: subsetI
    intro: subsetI Abrial_axiom1 [THEN subset_trans])
  done

text{*Lemma 3 of section 3.3*}
lemma eq_succ_upper: "[| n ∈ TFin S;  m ∈ TFin S;  m = succ S m |] ==> n ⊆ m"
  apply (erule TFin_induct)
   apply (drule TFin_subsetD)
     apply (assumption+, force, blast)
  done

text{*Property 3.3 of section 3.3*}
lemma equal_succ_Union: "m ∈ TFin S ==> (m = succ S m) = (m = Union(TFin S))"
  apply (rule iffI)
   apply (rule Union_upper [THEN equalityI])
    apply assumption
   apply (rule eq_succ_upper [THEN Union_least], assumption+)
  apply (erule ssubst)
  apply (rule Abrial_axiom1 [THEN equalityI])
  apply (blast del: subsetI intro: subsetI TFin_UnionI TFin.succI)
  done

subsection{*Hausdorff's Theorem: Every Set Contains a Maximal Chain.*}

text{*NB: We assume the partial ordering is @{text "⊆"},
 the subset relation!*}

lemma empty_set_mem_chain: "({} :: 'a set set) ∈ chain S"
  by (unfold chain_def chain_subset_def) simp

lemma super_subset_chain: "super S c ⊆ chain S"
  by (unfold super_def) blast

lemma maxchain_subset_chain: "maxchain S ⊆ chain S"
  by (unfold maxchain_def) blast

lemma mem_super_Ex: "c ∈ chain S - maxchain S ==> EX d. d ∈ super S c"
  by (unfold super_def maxchain_def) simp

lemma select_super:
     "c ∈ chain S - maxchain S ==> (\<some>c'. c': super S c): super S c"
  apply (erule mem_super_Ex [THEN exE])
  apply (rule someI2)
  apply simp+
  done

lemma select_not_equals:
     "c ∈ chain S - maxchain S ==> (\<some>c'. c': super S c) ≠ c"
  apply (rule notI)
  apply (drule select_super)
  apply (simp add: super_def less_le)
  done

lemma succI3: "c ∈ chain S - maxchain S ==> succ S c = (\<some>c'. c': super S c)"
  by (unfold succ_def) (blast intro!: if_not_P)

lemma succ_not_equals: "c ∈ chain S - maxchain S ==> succ S c ≠ c"
  apply (frule succI3)
  apply (simp (no_asm_simp))
  apply (rule select_not_equals, assumption)
  done

lemma TFin_chain_lemma4: "c ∈ TFin S ==> (c :: 'a set set): chain S"
  apply (erule TFin_induct)
   apply (simp add: succ_def select_super [THEN super_subset_chain[THEN subsetD]])
  apply (unfold chain_def chain_subset_def)
  apply (rule CollectI, safe)
   apply (drule bspec, assumption)
   apply (rule_tac [2] m1 = Xa and n1 = X in TFin_subset_linear [THEN disjE])
      apply blast+
  done

theorem Hausdorff: "∃c. (c :: 'a set set): maxchain S"
  apply (rule_tac x = "Union (TFin S)" in exI)
  apply (rule classical)
  apply (subgoal_tac "succ S (Union (TFin S)) = Union (TFin S) ")
   prefer 2
   apply (blast intro!: TFin_UnionI equal_succ_Union [THEN iffD2, symmetric])
  apply (cut_tac subset_refl [THEN TFin_UnionI, THEN TFin_chain_lemma4])
  apply (drule DiffI [THEN succ_not_equals], blast+)
  done


subsection{*Zorn's Lemma: If All Chains Have Upper Bounds Then
                               There Is  a Maximal Element*}

lemma chain_extend:
  "[| c ∈ chain S; z ∈ S; ∀x ∈ c. x ⊆ (z:: 'a set) |] ==> {z} Un c ∈ chain S"
by (unfold chain_def chain_subset_def) blast

lemma chain_Union_upper: "[| c ∈ chain S; x ∈ c |] ==> x ⊆ Union(c)"
by auto

lemma chain_ball_Union_upper: "c ∈ chain S ==> ∀x ∈ c. x ⊆ Union(c)"
by auto

lemma maxchain_Zorn:
  "[| c ∈ maxchain S; u ∈ S; Union(c) ⊆ u |] ==> Union(c) = u"
apply (rule ccontr)
apply (simp add: maxchain_def)
apply (erule conjE)
apply (subgoal_tac "({u} Un c) ∈ super S c")
 apply simp
apply (unfold super_def less_le)
apply (blast intro: chain_extend dest: chain_Union_upper)
done

theorem Zorn_Lemma:
  "∀c ∈ chain S. Union(c): S ==> ∃y ∈ S. ∀z ∈ S. y ⊆ z --> y = z"
apply (cut_tac Hausdorff maxchain_subset_chain)
apply (erule exE)
apply (drule subsetD, assumption)
apply (drule bspec, assumption)
apply (rule_tac x = "Union(c)" in bexI)
 apply (rule ballI, rule impI)
 apply (blast dest!: maxchain_Zorn, assumption)
done

subsection{*Alternative version of Zorn's Lemma*}

lemma Zorn_Lemma2:
  "∀c ∈ chain S. ∃y ∈ S. ∀x ∈ c. x ⊆ y
    ==> ∃y ∈ S. ∀x ∈ S. (y :: 'a set) ⊆ x --> y = x"
apply (cut_tac Hausdorff maxchain_subset_chain)
apply (erule exE)
apply (drule subsetD, assumption)
apply (drule bspec, assumption, erule bexE)
apply (rule_tac x = y in bexI)
 prefer 2 apply assumption
apply clarify
apply (rule ccontr)
apply (frule_tac z = x in chain_extend)
  apply (assumption, blast)
apply (unfold maxchain_def super_def less_le)
apply (blast elim!: equalityCE)
done

text{*Various other lemmas*}

lemma chainD: "[| c ∈ chain S; x ∈ c; y ∈ c |] ==> x ⊆ y | y ⊆ x"
by (unfold chain_def chain_subset_def) blast

lemma chainD2: "!!(c :: 'a set set). c ∈ chain S ==> c ⊆ S"
by (unfold chain_def) blast


(* Define globally? In Relation.thy? *)
definition Chain :: "('a*'a)set => 'a set set" where
"Chain r ≡ {A. ∀a∈A.∀b∈A. (a,b) : r ∨ (b,a) ∈ r}"

lemma mono_Chain: "r ⊆ s ==> Chain r ⊆ Chain s"
unfolding Chain_def by blast

text{* Zorn's lemma for partial orders: *}

lemma Zorns_po_lemma:
assumes po: "Partial_order r" and u: "∀C∈Chain r. ∃u∈Field r. ∀a∈C. (a,u):r"
shows "∃m∈Field r. ∀a∈Field r. (m,a):r --> a=m"
proof-
  have "Preorder r" using po by(simp add:partial_order_on_def)
--{* Mirror r in the set of subsets below (wrt r) elements of A*}
  let ?B = "%x. r^-1 `` {x}" let ?S = "?B ` Field r"
  have "∀C ∈ chain ?S. EX U:?S. ALL A:C. A⊆U"
  proof (auto simp:chain_def chain_subset_def)
    fix C assume 1: "C ⊆ ?S" and 2: "∀A∈C.∀B∈C. A⊆B | B⊆A"
    let ?A = "{x∈Field r. ∃M∈C. M = ?B x}"
    have "C = ?B ` ?A" using 1 by(auto simp: image_def)
    have "?A∈Chain r"
    proof (simp add:Chain_def, intro allI impI, elim conjE)
      fix a b
      assume "a ∈ Field r" "?B a ∈ C" "b ∈ Field r" "?B b ∈ C"
      hence "?B a ⊆ ?B b ∨ ?B b ⊆ ?B a" using 2 by simp
      thus "(a, b) ∈ r ∨ (b, a) ∈ r" using `Preorder r` `a:Field r` `b:Field r`
        by (simp add:subset_Image1_Image1_iff)
    qed
    then obtain u where uA: "u:Field r" "∀a∈?A. (a,u) : r" using u by auto
    have "∀A∈C. A ⊆ r^-1 `` {u}" (is "?P u")
    proof auto
      fix a B assume aB: "B:C" "a:B"
      with 1 obtain x where "x:Field r" "B = r^-1 `` {x}" by auto
      thus "(a,u) : r" using uA aB `Preorder r`
        by (simp add: preorder_on_def refl_on_def) (rule transD, blast+)
    qed
    thus "EX u:Field r. ?P u" using `u:Field r` by blast
  qed
  from Zorn_Lemma2[OF this]
  obtain m B where "m:Field r" "B = r^-1 `` {m}"
    "∀x∈Field r. B ⊆ r^-1 `` {x} --> B = r^-1 `` {x}"
    by auto
  hence "∀a∈Field r. (m, a) ∈ r --> a = m" using po `Preorder r` `m:Field r`
    by(auto simp:subset_Image1_Image1_iff Partial_order_eq_Image1_Image1_iff)
  thus ?thesis using `m:Field r` by blast
qed

(* The initial segment of a relation appears generally useful.
   Move to Relation.thy?
   Definition correct/most general?
   Naming?
*)
definition init_seg_of :: "(('a*'a)set * ('a*'a)set)set" where
"init_seg_of == {(r,s). r ⊆ s ∧ (∀a b c. (a,b):s ∧ (b,c):r --> (a,b):r)}"

abbreviation initialSegmentOf :: "('a*'a)set => ('a*'a)set => bool"
             (infix "initial'_segment'_of" 55) where
"r initial_segment_of s == (r,s):init_seg_of"

lemma refl_on_init_seg_of[simp]: "r initial_segment_of r"
by(simp add:init_seg_of_def)

lemma trans_init_seg_of:
  "r initial_segment_of s ==> s initial_segment_of t ==> r initial_segment_of t"
by (simp (no_asm_use) add: init_seg_of_def) (metis (no_types) in_mono order_trans)

lemma antisym_init_seg_of:
  "r initial_segment_of s ==> s initial_segment_of r ==> r=s"
unfolding init_seg_of_def by safe

lemma Chain_init_seg_of_Union:
  "R ∈ Chain init_seg_of ==> r∈R ==> r initial_segment_of \<Union>R"
by(simp add: init_seg_of_def Chain_def Ball_def) blast

lemma chain_subset_trans_Union:
  "chain⇘⊆⇙ R ==> ∀r∈R. trans r ==> trans(\<Union>R)"
apply(simp add:chain_subset_def)
apply(simp (no_asm_use) add:trans_def)
by (metis subsetD)

lemma chain_subset_antisym_Union:
  "chain⇘⊆⇙ R ==> ∀r∈R. antisym r ==> antisym(\<Union>R)"
by (simp add:chain_subset_def antisym_def) (metis subsetD)

lemma chain_subset_Total_Union:
assumes "chain⇘⊆⇙ R" "∀r∈R. Total r"
shows "Total (\<Union>R)"
proof (simp add: total_on_def Ball_def, auto del:disjCI)
  fix r s a b assume A: "r:R" "s:R" "a:Field r" "b:Field s" "a≠b"
  from `chain⇘⊆⇙ R` `r:R` `s:R` have "r⊆s ∨ s⊆r"
    by(simp add:chain_subset_def)
  thus "(∃r∈R. (a,b) ∈ r) ∨ (∃r∈R. (b,a) ∈ r)"
  proof
    assume "r⊆s" hence "(a,b):s ∨ (b,a):s" using assms(2) A
      by(simp add:total_on_def)(metis mono_Field subsetD)
    thus ?thesis using `s:R` by blast
  next
    assume "s⊆r" hence "(a,b):r ∨ (b,a):r" using assms(2) A
      by(simp add:total_on_def)(metis mono_Field subsetD)
    thus ?thesis using `r:R` by blast
  qed
qed

lemma wf_Union_wf_init_segs:
assumes "R ∈ Chain init_seg_of" and "∀r∈R. wf r" shows "wf(\<Union>R)"
proof(simp add:wf_iff_no_infinite_down_chain, rule ccontr, auto)
  fix f assume 1: "∀i. ∃r∈R. (f(Suc i), f i) ∈ r"
  then obtain r where "r:R" and "(f(Suc 0), f 0) : r" by auto
  { fix i have "(f(Suc i), f i) ∈ r"
    proof(induct i)
      case 0 show ?case by fact
    next
      case (Suc i)
      moreover obtain s where "s∈R" and "(f(Suc(Suc i)), f(Suc i)) ∈ s"
        using 1 by auto
      moreover hence "s initial_segment_of r ∨ r initial_segment_of s"
        using assms(1) `r:R` by(simp add: Chain_def)
      ultimately show ?case by(simp add:init_seg_of_def) blast
    qed
  }
  thus False using assms(2) `r:R`
    by(simp add:wf_iff_no_infinite_down_chain) blast
qed

lemma initial_segment_of_Diff:
  "p initial_segment_of q ==> p - s initial_segment_of q - s"
unfolding init_seg_of_def by blast

lemma Chain_inits_DiffI:
  "R ∈ Chain init_seg_of ==> {r - s |r. r ∈ R} ∈ Chain init_seg_of"
unfolding Chain_def by (blast intro: initial_segment_of_Diff)

theorem well_ordering: "∃r::('a*'a)set. Well_order r ∧ Field r = UNIV"
proof-
-- {*The initial segment relation on well-orders: *}
  let ?WO = "{r::('a*'a)set. Well_order r}"
  def I ≡ "init_seg_of ∩ ?WO × ?WO"
  have I_init: "I ⊆ init_seg_of" by(simp add: I_def)
  hence subch: "!!R. R : Chain I ==> chain⇘⊆⇙ R"
    by(auto simp:init_seg_of_def chain_subset_def Chain_def)
  have Chain_wo: "!!R r. R ∈ Chain I ==> r ∈ R ==> Well_order r"
    by(simp add:Chain_def I_def) blast
  have FI: "Field I = ?WO" by(auto simp add:I_def init_seg_of_def Field_def)
  hence 0: "Partial_order I"
    by(auto simp: partial_order_on_def preorder_on_def antisym_def antisym_init_seg_of refl_on_def trans_def I_def elim!: trans_init_seg_of)
-- {*I-chains have upper bounds in ?WO wrt I: their Union*}
  { fix R assume "R ∈ Chain I"
    hence Ris: "R ∈ Chain init_seg_of" using mono_Chain[OF I_init] by blast
    have subch: "chain⇘⊆⇙ R" using `R : Chain I` I_init
      by(auto simp:init_seg_of_def chain_subset_def Chain_def)
    have "∀r∈R. Refl r" "∀r∈R. trans r" "∀r∈R. antisym r" "∀r∈R. Total r"
         "∀r∈R. wf(r-Id)"
      using Chain_wo[OF `R ∈ Chain I`] by(simp_all add:order_on_defs)
    have "Refl (\<Union>R)" using `∀r∈R. Refl r` by(auto simp:refl_on_def)
    moreover have "trans (\<Union>R)"
      by(rule chain_subset_trans_Union[OF subch `∀r∈R. trans r`])
    moreover have "antisym(\<Union>R)"
      by(rule chain_subset_antisym_Union[OF subch `∀r∈R. antisym r`])
    moreover have "Total (\<Union>R)"
      by(rule chain_subset_Total_Union[OF subch `∀r∈R. Total r`])
    moreover have "wf((\<Union>R)-Id)"
    proof-
      have "(\<Union>R)-Id = \<Union>{r-Id|r. r ∈ R}" by blast
      with `∀r∈R. wf(r-Id)` wf_Union_wf_init_segs[OF Chain_inits_DiffI[OF Ris]]
      show ?thesis by (simp (no_asm_simp)) blast
    qed
    ultimately have "Well_order (\<Union>R)" by(simp add:order_on_defs)
    moreover have "∀r ∈ R. r initial_segment_of \<Union>R" using Ris
      by(simp add: Chain_init_seg_of_Union)
    ultimately have "\<Union>R : ?WO ∧ (∀r∈R. (r,\<Union>R) : I)"
      using mono_Chain[OF I_init] `R ∈ Chain I`
      by(simp (no_asm) add:I_def del:Field_Union)(metis Chain_wo)
  }
  hence 1: "∀R ∈ Chain I. ∃u∈Field I. ∀r∈R. (r,u) : I" by (subst FI) blast
--{*Zorn's Lemma yields a maximal well-order m:*}
  then obtain m::"('a*'a)set" where "Well_order m" and
    max: "∀r. Well_order r ∧ (m,r):I --> r=m"
    using Zorns_po_lemma[OF 0 1] by (auto simp:FI)
--{*Now show by contradiction that m covers the whole type:*}
  { fix x::'a assume "x ∉ Field m"
--{*We assume that x is not covered and extend m at the top with x*}
    have "m ≠ {}"
    proof
      assume "m={}"
      moreover have "Well_order {(x,x)}"
        by(simp add:order_on_defs refl_on_def trans_def antisym_def total_on_def Field_def Domain_unfold Domain_converse [symmetric])
      ultimately show False using max
        by (auto simp:I_def init_seg_of_def simp del:Field_insert)
    qed
    hence "Field m ≠ {}" by(auto simp:Field_def)
    moreover have "wf(m-Id)" using `Well_order m`
      by(simp add:well_order_on_def)
--{*The extension of m by x:*}
    let ?s = "{(a,x)|a. a : Field m}" let ?m = "insert (x,x) m Un ?s"
    have Fm: "Field ?m = insert x (Field m)"
      apply(simp add:Field_insert Field_Un)
      unfolding Field_def by auto
    have "Refl m" "trans m" "antisym m" "Total m" "wf(m-Id)"
      using `Well_order m` by(simp_all add:order_on_defs)
--{*We show that the extension is a well-order*}
    have "Refl ?m" using `Refl m` Fm by(auto simp:refl_on_def)
    moreover have "trans ?m" using `trans m` `x ∉ Field m`
      unfolding trans_def Field_def Domain_unfold Domain_converse [symmetric] by blast
    moreover have "antisym ?m" using `antisym m` `x ∉ Field m`
      unfolding antisym_def Field_def Domain_unfold Domain_converse [symmetric] by blast
    moreover have "Total ?m" using `Total m` Fm by(auto simp: total_on_def)
    moreover have "wf(?m-Id)"
    proof-
      have "wf ?s" using `x ∉ Field m`
        by(simp add:wf_eq_minimal Field_def Domain_unfold Domain_converse [symmetric]) metis
      thus ?thesis using `wf(m-Id)` `x ∉ Field m`
        wf_subset[OF `wf ?s` Diff_subset]
        by (fastforce intro!: wf_Un simp add: Un_Diff Field_def)
    qed
    ultimately have "Well_order ?m" by(simp add:order_on_defs)
--{*We show that the extension is above m*}
    moreover hence "(m,?m) : I" using `Well_order m` `x ∉ Field m`
      by(fastforce simp:I_def init_seg_of_def Field_def Domain_unfold Domain_converse [symmetric])
    ultimately
--{*This contradicts maximality of m:*}
    have False using max `x ∉ Field m` unfolding Field_def by blast
  }
  hence "Field m = UNIV" by auto
  moreover with `Well_order m` have "Well_order m" by simp
  ultimately show ?thesis by blast
qed

corollary well_order_on: "∃r::('a*'a)set. well_order_on A r"
proof -
  obtain r::"('a*'a)set" where wo: "Well_order r" and univ: "Field r = UNIV"
    using well_ordering[where 'a = "'a"] by blast
  let ?r = "{(x,y). x:A & y:A & (x,y):r}"
  have 1: "Field ?r = A" using wo univ
    by(fastforce simp: Field_def Domain_unfold Domain_converse [symmetric] order_on_defs refl_on_def)
  have "Refl r" "trans r" "antisym r" "Total r" "wf(r-Id)"
    using `Well_order r` by(simp_all add:order_on_defs)
  have "Refl ?r" using `Refl r` by(auto simp:refl_on_def 1 univ)
  moreover have "trans ?r" using `trans r`
    unfolding trans_def by blast
  moreover have "antisym ?r" using `antisym r`
    unfolding antisym_def by blast
  moreover have "Total ?r" using `Total r` by(simp add:total_on_def 1 univ)
  moreover have "wf(?r - Id)" by(rule wf_subset[OF `wf(r-Id)`]) blast
  ultimately have "Well_order ?r" by(simp add:order_on_defs)
  with 1 show ?thesis by metis
qed

end