Theory BDT

section‹Binary Decision Trees›
theory BDT
imports Bool_Func
begin

text‹
We first define all operations and properties on binary decision trees.
This has the advantage that we can use a simple, structurally defined type
and the disadvantage that we cannot represent sharing.
›

datatype 'a ifex = Trueif | Falseif | IF 'a "'a ifex" "'a ifex"

text‹The type is the same as in Boolean Expression Checkers by Nipkow~\<^cite>‹"Boolean_Expression_Checkers-AFP"›.
Internally, Boolean Expression Checkers transforms the boolean expressions to reduced BDTs of this type.
Tests like being tautology testing are then trivial.
›

fun val_ifex :: "'a ifex ⇒ ('a ⇒ bool) ⇒ bool" where
  "val_ifex Trueif s = True" |
  "val_ifex Falseif s = False" |
  "val_ifex (IF n t1 t2) s = (if s n then val_ifex t1 s else val_ifex t2 s)"

fun ifex_vars :: "('a :: linorder) ifex ⇒ 'a list" where
  "ifex_vars (IF v t e) =  v # ifex_vars t @ ifex_vars e" |
  "ifex_vars Trueif = []" |
  "ifex_vars Falseif = []"

abbreviation "ifex_var_set a ≡ set (ifex_vars a)"

fun ifex_ordered :: "('a::linorder) ifex ⇒ bool" where
  "ifex_ordered (IF v t e) = ((∀tv ∈ (ifex_var_set t ∪ ifex_var_set e). v < tv)
                              ∧ ifex_ordered t ∧ ifex_ordered e)" |
  "ifex_ordered Trueif = True" |
  "ifex_ordered Falseif = True"

fun ifex_minimal :: "('a::linorder) ifex ⇒ bool" where
  "ifex_minimal (IF v t e) ⟷ t ≠ e ∧ ifex_minimal t ∧ ifex_minimal e" |
  "ifex_minimal Trueif = True" |
  "ifex_minimal Falseif = True"

abbreviation ro_ifex where "ro_ifex t ≡ ifex_ordered t ∧ ifex_minimal t"

definition bf_ifex_rel where
  "bf_ifex_rel = {(a,b). (∀ass. a ass ⟷ val_ifex b ass) ∧ ro_ifex b}"

lemma ifex_var_noinfluence: "x ∉ ifex_var_set b ⟹ val_ifex b (ass(x:=val)) = val_ifex b ass"
  by (induction b, auto)

lemma roifex_var_not_in_subtree:
  assumes "ro_ifex b" and "b = IF v t e" 
  shows "v ∉ ifex_var_set t" and "v ∉ ifex_var_set e"
using assms by (induction, auto)

lemma roifex_set_var_subtree: 
  assumes "ro_ifex b" and "b = IF v t e"
  shows "val_ifex b (ass(v:=True)) = val_ifex t ass" 
        "val_ifex b (ass(v:=False)) = val_ifex e ass"
  using assms by (auto intro!: ifex_var_noinfluence dest: roifex_var_not_in_subtree)

lemma roifex_Trueif_unique: "ro_ifex b ⟹ ∀ass. val_ifex b ass ⟹ b = Trueif"
proof(induction b)
  case (IF v b1 b2) with roifex_set_var_subtree[OF ‹ro_ifex (IF v b1 b2)›] show ?case by force
qed(auto)

lemma roifex_Falseif_unique: "ro_ifex b ⟹ ∀ass. ¬ val_ifex b ass ⟹ b = Falseif"
proof(induction b)
  case (IF v b1 b2) with roifex_set_var_subtree[OF ‹ro_ifex (IF v b1 b2)›, of v b1 b2] show ?case
    by fastforce
qed(auto)

lemma "(f, b) ∈ bf_ifex_rel ⟹  b = Trueif ⟷ f = (λ_. True)"
  unfolding bf_ifex_rel_def using roifex_Trueif_unique by auto

lemma "(f, b) ∈ bf_ifex_rel ⟹  b = Falseif ⟷ f = (λ_. False)"
  unfolding bf_ifex_rel_def using roifex_Falseif_unique by auto

lemma ifex_ordered_not_part: "ifex_ordered  b ⟹ b = IF v b1 b2 ⟹ w < v ⟹ w ∉ ifex_var_set b"
  using less_asym by fastforce

lemma ro_ifex_unique: "ro_ifex x ⟹ ro_ifex y ⟹ (⋀ass. val_ifex x ass = val_ifex y ass) ⟹ x = y"
 proof(induction x arbitrary: y)
  case (IF xv xb1 xb2) note IFind = IF 
    from ‹ro_ifex (IF xv xb1 xb2)›  ‹ro_ifex y› ‹⋀ass. val_ifex (IF xv xb1 xb2) ass = val_ifex y ass›
      show ?case
        proof(induction y)
          case (IF yv yb1 yb2)
            obtain x where x_def: "x = IF xv xb1 xb2" by simp
            obtain y' where y'_def: "y' = IF yv yb1 yb2" by simp
            from y'_def x_def IFind IF have 0: "ro_ifex xb1" "ro_ifex xb2" "ro_ifex yb1" 
                                               "ro_ifex yb2" "ro_ifex x" "ro_ifex y'" by auto
            from IF IFind x_def y'_def have 1: "⋀ass. val_ifex x ass = val_ifex y' ass" by simp
            show ?case
              proof(cases "xv = yv")
                case True
      have "xb1 = yb1"
        by (auto intro: IFind simp add: 0 1 True roifex_set_var_subtree[OF _ y'_def]
                                        roifex_set_var_subtree[OF _ x_def, symmetric])
      moreover have "xb2 = yb2"
        by (auto intro: IFind simp add: 0 1 True roifex_set_var_subtree[OF _ y'_def]
                                        roifex_set_var_subtree[OF _ x_def, symmetric])
      ultimately show ?thesis using True by simp
    next
    case False note uneq = False show ?thesis
      proof(cases "xv < yv")
        case True
          from ifex_ordered_not_part[OF _ y'_def True] ifex_var_noinfluence[of xv y' _ "True"]
               0(6) roifex_set_var_subtree(1)[OF 0(5) x_def] 1
             have 5: "⋀ass. val_ifex xb1 ass = val_ifex x ass" by blast
          from 0(5) ifex_var_noinfluence[of xv xb1 _ "False"] 
                    ifex_var_noinfluence[of xv xb2 _ "False"] 
               x_def
            have "⋀ass. val_ifex xb1 (ass(xv := False)) = val_ifex xb1 ass"
                 "⋀ass. val_ifex xb2 (ass(xv := False)) = val_ifex xb2 ass" by auto
          from 5 this roifex_set_var_subtree(2)[OF 0(5) x_def]
            have "⋀ass. val_ifex xb1 ass = val_ifex xb2 ass" by presburger
          from IFind(1)[OF 0(1) 0(2)] this IFind(3) have "False" by auto
          from this show ?thesis ..
      next
        case False
          from this uneq have 6: "yv < xv" by auto
          from ifex_ordered_not_part[OF _ x_def this]
                     ifex_var_noinfluence[of yv x] 0(5)
             have  "⋀ass val. val_ifex x (ass(yv := val)) = val_ifex x ass" 
                   "⋀ass val. val_ifex x (ass(yv := val)) =  val_ifex x ass" by auto
          from this roifex_set_var_subtree[OF 0(5) x_def]
            have "⋀ass val. val_ifex x (ass(xv := True, yv := val)) = val_ifex xb1 ass"
                 "⋀ass val. val_ifex x (ass(xv := False, yv := val)) = val_ifex xb2 ass" by blast+
          from ifex_ordered_not_part[OF _ x_def 6] 0(5) ifex_var_noinfluence[of yv x] 1
               roifex_set_var_subtree[OF 0(6) y'_def]
            have "⋀ass val. val_ifex x ass = val_ifex yb1 ass"
                 "⋀ass val. val_ifex x ass = val_ifex yb2 ass" by blast+
          from this IF(1,2) x_def 0(5) y'_def 0(6) have "x = yb1" "x = yb2" by fastforce+
          from this have "yb1 = yb2" by auto
          from 0(6) y'_def this have "False" by auto
          thus ?thesis ..
      qed
  qed
qed (fastforce intro: roifex_Falseif_unique roifex_Trueif_unique)+
qed (fastforce intro: roifex_Falseif_unique[symmetric] roifex_Trueif_unique[symmetric])+

theorem bf_ifex_rel_single: "single_valued bf_ifex_rel" "single_valued (bf_ifex_rel¯)"
  unfolding single_valued_def bf_ifex_rel_def using ro_ifex_unique by auto

lemma bf_ifex_eq: "(af, at) ∈ bf_ifex_rel ⟹ (bf, bt) ∈ bf_ifex_rel ⟹ (af = bf) ⟷ (at = bt)"
  unfolding bf_ifex_rel_def using ro_ifex_unique by auto

lemma nonempty_if_var_set: "ifex_vars (IF v t e) ≠ []" by auto

fun restrict where
  "restrict (IF v t e) var val = (let rt = restrict t var val; re = restrict e var val in
                   (if v = var then (if val then rt else re) else (IF v rt re)))" |
  "restrict i _ _ = i"

declare Let_def[simp]

lemma not_element_restrict: "var ∉ ifex_var_set (restrict b var val)"
  by (induction b) auto

lemma restrict_assignment: "val_ifex b (ass(var := val)) ⟷ val_ifex (restrict b var val) ass"
  by (induction b) auto

lemma restrict_variables_subset: "ifex_var_set (restrict b var val) ⊆ ifex_var_set b"
  by (induction b) auto

lemma restrict_ifex_ordered_invar: "ifex_ordered b ⟹ ifex_ordered (restrict b var val)"
  using restrict_variables_subset by (induction b) (fastforce)+

lemma restrict_val_invar: "∀ass. a ass = val_ifex b ass ⟹
                           (bf_restrict var val a) ass = val_ifex (restrict b var val) ass"
  unfolding bf_restrict_def using restrict_assignment by simp

lemma restrict_untouched_id: "x ∉ ifex_var_set t ⟹ restrict t x val = t" (* inversion should hold, too… *)
proof(induction t)
  case (IF v t e)
  from IF.prems have "x ∉ ifex_var_set t" "x ∉ ifex_var_set e" by simp_all
  note mIH = IF.IH(1)[OF this(1)] IF.IH(2)[OF this(2)]
  from IF.prems have "x ≠ v" by simp
  thus ?case unfolding restrict.simps Let_def mIH  by simp
qed simp_all

fun ifex_top_var :: "'a ifex ⇒ 'a option" where
  "ifex_top_var (IF v t e) = Some v" |
  "ifex_top_var _ = None"

fun restrict_top :: "('a :: linorder) ifex ⇒ 'a ⇒ bool ⇒ 'a ifex" where
  "restrict_top (IF v t e) var val = (if v = var then (if val then t else e) else (IF v t e))" |
  "restrict_top i _ _ = i"

(* the following four are might be useful eventually… *)
lemma restrict_top_id: "ifex_ordered e ⟹ ifex_top_var e = Some v ⟹ v' < v ⟹ restrict_top e v' val = e"
  by(induction e) auto

lemma restrict_id: "ifex_ordered e ⟹ ifex_top_var e = Some v ⟹ v' < v ⟹ restrict e v' val = e"
  proof(induction e arbitrary: v)
    case (IF w e1 e2) thus ?case by (cases e1; cases e2; force)
  qed(auto)

lemma restrict_top_IF_id: "ifex_ordered (IF v t e) ⟹ v' < v ⟹ restrict_top (IF v t e) v' val = (IF v t e)"
  using restrict_top_id by auto

lemma restrict_IF_id: assumes o: "ifex_ordered (IF v t e)" assumes le: "v' < v"
  shows "restrict (IF v t e) v' val = (IF v t e)"
  using restrict_id[OF o, unfolded ifex_top_var.simps, OF refl le, of val] .

lemma restrict_top_eq: "ifex_ordered (IF v t e) ⟹ restrict (IF v t e) v val = restrict_top (IF v t e) v val"
  using restrict_untouched_id by auto

lemma restrict_top_ifex_ordered_invar: "ifex_ordered b ⟹ ifex_ordered (restrict_top b var val)"
  by (induction b) simp_all

fun lowest_tops :: "('a :: linorder) ifex list ⇒ 'a option" where
  "lowest_tops [] = None" |
  "lowest_tops ((IF v _ _)#r) = Some (case lowest_tops r of Some u ⇒ (min u v) | None ⇒ v)" |
  "lowest_tops (_#r) = lowest_tops r"

lemma lowest_tops_NoneD: "lowest_tops k = None ⟹ (¬(∃v t e. ((IF v t e) ∈ set k)))"
  by (induction k rule: lowest_tops.induct) simp_all

lemma lowest_tops_in: "lowest_tops k = Some l ⟹ l ∈ set (concat (map ifex_vars k))"
  by(induction k rule: lowest_tops.induct) (simp_all split: option.splits if_splits add: min_def)

definition "IFC v t e ≡ (if t = e then t else IF v t e)"

function ifex_ite :: "'a ifex ⇒ 'a ifex ⇒ 'a ifex ⇒ ('a :: linorder) ifex" where
  "ifex_ite i t e = (case lowest_tops [i, t, e] of Some x ⇒ 
                         (IFC x (ifex_ite (restrict_top i x True) (restrict_top t x True) (restrict_top e x True))
                               (ifex_ite (restrict_top i x False) (restrict_top t x False) (restrict_top e x False)))
                     | None ⇒ (case i of Trueif ⇒ t | Falseif ⇒ e))"
by pat_completeness auto

lemma restrict_size_le: "size (restrict_top k var val) ≤ size k"
  by (induction k, auto)

lemma restrict_size_less: "ifex_top_var k = Some var ⟹ size (restrict_top k var val) < size k"
  by (induction k, auto)

lemma lowest_tops_cases:
"lowest_tops [i, t, e] = Some var ⟹ ifex_top_var i = Some var ∨ ifex_top_var t
                                      = Some var ∨ ifex_top_var e = Some var"
  by ((cases i; cases t; cases e), auto simp add: min_def)

lemma lowest_tops_lowest: "lowest_tops es = Some a ⟹ e ∈ set es ⟹ ifex_ordered e ⟹ v ∈ ifex_var_set e ⟹ a ≤ v"
proof (induction arbitrary: a rule: lowest_tops.induct)
  case 2 thus ?case
  proof(cases e)
    case IF with 2 show ?thesis
     apply (simp add: min_def Ball_def less_imp_le split: if_splits option.splits)
       apply (meson less_imp_le lowest_tops_NoneD order_refl)
      by fastforce+
  qed simp+
qed fastforce+

lemma termlemma2: "lowest_tops [i, t, e] = Some xa ⟹
  (size (restrict_top i xa val) + size (restrict_top t xa val) + size (restrict_top e xa val)) <
  (size i + size t + size e)"
  using restrict_size_le[of i xa val] restrict_size_le[of t xa val]  restrict_size_le[of e xa val]
  by (auto dest!: lowest_tops_cases restrict_size_less[of _ _ val])

lemma termlemma: "lowest_tops [i, t, e] = Some xa ⟹
       (case (restrict_top i xa val, restrict_top t xa val, restrict_top e xa val) of 
             (i, t, e) ⇒ size i + size t + size e) <
       (case (i, t, e) of (i, t, e) ⇒ size i + size t + size e)"
  using termlemma2 by fast

termination ifex_ite
  by (relation "measure (λ(i,t,e). size i + size t + size e)", rule wf_measure, unfold in_measure) 
     (simp_all only: termlemma)


definition "const x _ = x" (* inspired by Haskell *)
declare const_def[simp]
lemma rel_true_false: "(a, Trueif) ∈ bf_ifex_rel ⟹ a = const True" "(a, Falseif) ∈ bf_ifex_rel ⟹ a = const False"
  unfolding fun_eq_iff const_def
  unfolding bf_ifex_rel_def
  by simp_all

lemma rel_if: "(a, IF v t e) ∈ bf_ifex_rel ⟹ (ta, t) ∈ bf_ifex_rel ⟹ (ea, e) ∈ bf_ifex_rel ⟹ a = (λas. if as v then ta as else ea as)"
  unfolding fun_eq_iff const_def
  unfolding bf_ifex_rel_def
  by simp_all


lemma ifex_ordered_implied: "(a, b) ∈ bf_ifex_rel ⟹ ifex_ordered b" unfolding bf_ifex_rel_def by simp
lemma ifex_minimal_implied: "(a, b) ∈ bf_ifex_rel ⟹ ifex_minimal b" unfolding bf_ifex_rel_def by simp


lemma ifex_ite_induct2[case_names Trueif Falseif IF]: "
  (⋀i t e. lowest_tops [i, t, e] = None ⟹ i = Trueif ⟹ sentence i t e) ⟹
  (⋀i t e. lowest_tops [i, t, e] = None ⟹ i = Falseif ⟹ sentence i t e) ⟹
  (⋀i t e a. sentence (restrict_top i a True) (restrict_top t a True) (restrict_top e a True) ⟹
             sentence (restrict_top i a False) (restrict_top t a False) (restrict_top e a False) ⟹
   lowest_tops [i, t, e] = Some a ⟹ sentence i t e) ⟹ sentence i t e"
proof(induction i t e rule: ifex_ite.induct, goal_cases)
  case (1 i t e) show ?case
  proof(cases "lowest_tops [i, t, e]")
    case None thus ?thesis by (cases i) (auto intro: 1)
  next
    case (Some a) thus ?thesis by(auto intro: 1)
  qed
qed

lemma ifex_ite_induct[case_names Trueif Falseif IF]: "
  (⋀i t e. lowest_tops [i, t, e] = None ⟹ i = Trueif ⟹ P i t e) ⟹
  (⋀i t e. lowest_tops [i, t, e] = None ⟹ i = Falseif ⟹ P i t e) ⟹
  (⋀i t e a. (⋀val. P (restrict_top i a val) (restrict_top t a val) (restrict_top e a val)) ⟹ 
   lowest_tops [i, t, e] = Some a ⟹ P i t e) ⟹ P i t e"
proof(induction i t e rule: ifex_ite_induct2)
  case (IF i t e a)
  have "⋀val. (P (restrict_top i a val) (restrict_top t a val) (restrict_top e a val))"
    by (case_tac val) (clarsimp, blast intro: IF)+
  with IF show ?case by blast
qed blast+

lemma restrict_top_subset: "x ∈ ifex_var_set (restrict_top i vr vl) ⟹ x ∈ ifex_var_set i"
  by(induction i) (simp_all split: if_splits)

lemma ifex_vars_subset: "x ∈ ifex_var_set (ifex_ite i t e) ⟹ (x ∈ ifex_var_set i) ∨ (x ∈ ifex_var_set t) ∨ (x ∈ ifex_var_set e)"
proof(induction rule: ifex_ite_induct2)
  case (IF i t e a)
  have "x ∈ {x. x = a} ∨ x ∈ (ifex_var_set (ifex_ite (restrict_top i a True) (restrict_top t a True) (restrict_top e a True))) ∨ x ∈ (ifex_var_set (ifex_ite (restrict_top i a False) (restrict_top t a False) (restrict_top e a False)))"
    using IF by(simp add: IFC_def split: if_splits) 
  hence "x = a ∨
    x ∈ (ifex_var_set (restrict_top i a True )) ∨ x ∈ (ifex_var_set (restrict_top t a True )) ∨ x ∈ (ifex_var_set (restrict_top e a True )) ∨
    x ∈ (ifex_var_set (restrict_top i a False)) ∨ x ∈ (ifex_var_set (restrict_top t a False)) ∨ x ∈ (ifex_var_set (restrict_top e a False))"
  using IF by blast
  thus ?case
    using restrict_top_subset apply -
    apply(erule disjE)
      subgoal using lowest_tops_in[OF IF(3)] apply(simp only: set_concat set_map set_simps) by blast
    by blast
qed simp_all

lemma three_ins: "i ∈ set [i, t, e]" "t ∈ set [i, t, e]" "e ∈ set [i, t, e]" by simp_all

lemma hlp3: "lowest_tops (IF v uu uv # r) ≠ lowest_tops r ⟹ lowest_tops (IF v uu uv # r) = Some v"
  by(simp add: min_def split: option.splits if_splits)

lemma hlp2: "IF vi vt ve ∈ set is ⟹ lowest_tops is = Some a ⟹ a ≤ vi"
  apply(induction "is" arbitrary: vt ve a rule: lowest_tops.induct)
    prefer 2
    subgoal
      apply(auto simp add: min_def split: if_splits option.splits dest: lowest_tops_NoneD)
      by (meson le_cases order_trans)
    by (auto)

lemma hlp1: "i ∈ set is ⟹ lowest_tops is = Some a ⟹ ifex_ordered i ⟹ a ∉ (ifex_var_set (restrict_top i a val))"
proof(rule ccontr, unfold not_not, goal_cases)
  case 1
  from 1(4) obtain vi vt ve where vi: "i = IF vi vt ve" by(cases i) simp_all
  with 1 have ne: "vi ≠ a" by(simp split: if_splits) blast+
  moreover have "vi ≤ a" using 1(3,4) proof(-,goal_cases)
    case 1
    hence "a ∈ (ifex_var_set vt) ∨ a ∈ (ifex_var_set ve)" using ne by(simp add: vi)
    thus ?case using ‹ifex_ordered i› vi using less_imp_le by auto
    qed
  moreover have "a ≤ vi" using 1(1) unfolding vi using 1(2) hlp2 by metis
  ultimately show False by simp
qed

lemma order_ifex_ite_invar: "ifex_ordered i ⟹ ifex_ordered t ⟹ ifex_ordered e ⟹ ifex_ordered (ifex_ite i t e)"
proof(induction i t e rule: ifex_ite_induct)
  case (IF i t e) note goal1 = IF
  note l = restrict_top_ifex_ordered_invar
  note l[OF goal1(3)] l[OF goal1(4)] l[OF goal1(5)]
  note mIH = goal1(1)[OF this]
  note blubb = lowest_tops_lowest[OF goal1(2) _ _ restrict_top_subset]
  show ?case using mIH 
  by (subst ifex_ite.simps,
    auto simp del: ifex_ite.simps
      simp add: IFC_def goal1(2) hlp1[OF three_ins(1) goal1(2) goal1(3)] hlp1[OF three_ins(2) goal1(2) goal1(4)] hlp1[OF three_ins(3) goal1(2) goal1(5)] 
      dest: ifex_vars_subset blubb[OF three_ins(1) goal1(3)] blubb[OF three_ins(2) goal1(4)] blubb[OF three_ins(3) goal1(5)] 
      intro!: le_neq_trans)
qed simp_all

lemma ifc_split: "P (IFC v t e) ⟷ ((t = e) ⟶ P t) ∧ (t ≠ e ⟶ P (IF v t e))"
  unfolding IFC_def by simp

lemma restrict_top_ifex_minimal_invar: "ifex_minimal i ⟹ ifex_minimal (restrict_top i a val)"
  by(induction i) simp_all

lemma minimal_ifex_ite_invar: "ifex_minimal i ⟹ ifex_minimal t ⟹ ifex_minimal e ⟹ ifex_minimal (ifex_ite i t e)"
  by(induction i t e rule: ifex_ite_induct) (simp_all split: ifc_split option.split add: restrict_top_ifex_minimal_invar)

lemma restrict_top_bf: "i ∈ set is ⟹ lowest_tops is = Some vr ⟹
  ifex_ordered i ⟹ (⋀ass. fi ass = val_ifex i ass) ⟹ val_ifex (restrict_top i vr vl) ass = bf_restrict vr vl fi ass"
proof(cases i, goal_cases)
  case (3 x31 x32 x33)  note goal3 = 3
  have rr: "restrict_top i vr vl = restrict i vr vl" 
  proof(cases "x31 = vr")
    case True
    note uf = restrict_top_eq[OF goal3(3)[unfolded goal3(5)], symmetric, unfolded goal3(5)[symmetric], unfolded True]
    thus ?thesis .
  next
    case False
    have 1: "restrict_top i vr vl = i" by (simp add: False goal3(5))
    have "vr < x31" using le_neq_trans[OF hlp2[OF goal3(1)[unfolded goal3(5)] goal3(2)] False[symmetric]] by blast
    with goal3(3,5) have 2: "restrict i vr vl = i" using restrict_IF_id by blast
    show ?thesis unfolding 1 2 ..
  qed
  show ?case unfolding rr by(simp add: goal3(4) restrict_val_invar[symmetric])
qed (simp_all add: bf_restrict_def)

lemma val_ifex_ite: "
  (⋀ass. fi ass = val_ifex i ass) ⟹
  (⋀ass. ft ass = val_ifex t ass) ⟹
  (⋀ass. fe ass = val_ifex e ass) ⟹
  ifex_ordered i ⟹ ifex_ordered t ⟹ ifex_ordered e ⟹
  (bf_ite fi ft fe) ass = val_ifex (ifex_ite i t e) ass"
proof(induction i t e arbitrary: fi ft fe rule: ifex_ite_induct)
  case (IF i t e a)
  note mIH = IF(1)[OF refl refl refl
    restrict_top_ifex_ordered_invar[OF IF(6)]
    restrict_top_ifex_ordered_invar[OF IF(7)]
    restrict_top_ifex_ordered_invar[OF IF(8)], symmetric]
  note uf1 = restrict_top_bf[OF three_ins(1) IF(2) ‹ifex_ordered i›  IF(3)]
             restrict_top_bf[OF three_ins(2) IF(2) ‹ifex_ordered t›  IF(4)]
             restrict_top_bf[OF three_ins(3) IF(2) ‹ifex_ordered e›  IF(5)]
  show ?case
    by(rule trans[OF brace90shannon[where i=a]])
      (auto simp: restrict_top_ifex_ordered_invar IF(1,2,6-8) uf1 mIH bf_ite_def[of "λl. l a"]
            split: ifc_split)
qed (simp add: bf_ite_def bf_ifex_rel_def)+

theorem ifex_ite_rel_bf: "
  (fi,i) ∈ bf_ifex_rel ⟹
  (ft,t) ∈ bf_ifex_rel ⟹
  (fe,e) ∈ bf_ifex_rel ⟹
  ((bf_ite fi ft fe), (ifex_ite i t e)) ∈ bf_ifex_rel"
by (auto simp add:  bf_ifex_rel_def order_ifex_ite_invar minimal_ifex_ite_invar val_ifex_ite
         simp del: ifex_ite.simps)

definition param_opt where "param_opt i t e =
  (if i = Trueif then Some t else
   if i = Falseif then Some e else
   if t = Trueif ∧ e = Falseif then Some i else
   if t = e then Some t else
   if e = Trueif ∧ i = t then Some Trueif else
   if t = Falseif ∧ i = e then Some Falseif else
   None)"

lemma param_opt_ifex_ite_eq: "ro_ifex i ⟹ ro_ifex t ⟹ ro_ifex e ⟹
       param_opt i t e = Some r ⟹ r = ifex_ite i t e"
  apply(rule ro_ifex_unique)
   subgoal by (subst (asm) param_opt_def) (simp split: if_split_asm)
   subgoal using order_ifex_ite_invar minimal_ifex_ite_invar by (blast)
   by (subst val_ifex_ite[symmetric])
      (auto split: if_split_asm simp add: bf_ite_def param_opt_def val_ifex_ite[symmetric])


function ifex_ite_opt :: "'a ifex ⇒ 'a ifex ⇒ 'a ifex ⇒ ('a :: linorder) ifex" where
  "ifex_ite_opt i t e = (case param_opt i t e of Some b ⇒ b | None ⇒
                        (case lowest_tops [i, t, e] of Some x ⇒ 
                         (IFC x (ifex_ite_opt (restrict_top i x True) (restrict_top t x True)
                                              (restrict_top e x True))
                                (ifex_ite_opt (restrict_top i x False) (restrict_top t x False)
                                              (restrict_top e x False)))
                     | None ⇒ (case i of Trueif ⇒ t | Falseif ⇒ e)))"
by pat_completeness auto

termination ifex_ite_opt
  by (relation "measure (λ(i,t,e). size i + size t + size e)", rule wf_measure, unfold in_measure)
     (simp_all only: termlemma)

lemma ifex_ite_opt_eq: "
  ro_ifex i ⟹ ro_ifex t ⟹ ro_ifex e ⟹ ifex_ite_opt i t e = ifex_ite i t e"
  apply(induction i t e rule: ifex_ite_opt.induct)
  apply(subst ifex_ite_opt.simps)
  apply(rename_tac i t e)
  apply(case_tac "∃r. param_opt i t e = Some r")
   subgoal
    apply(simp del: ifex_ite.simps restrict_top.simps lowest_tops.simps)
    apply(rule param_opt_ifex_ite_eq)
    by (auto simp add: bf_ifex_rel_def)
   subgoal for i t e
    apply(clarsimp simp del: restrict_top.simps ifex_ite.simps ifex_ite_opt.simps)
    apply(cases "lowest_tops [i,t,e] = None")
     subgoal by clarsimp
     subgoal
      apply(clarsimp simp del: restrict_top.simps ifex_ite.simps ifex_ite_opt.simps)
      apply(subst ifex_ite.simps)
      apply(rename_tac y)
      apply(subgoal_tac "(ifex_ite_opt (restrict_top i y True) (restrict_top t y True) (restrict_top e y True)) =
                         (ifex_ite (restrict_top i y True) (restrict_top t y True) (restrict_top e y True))")
      apply(subgoal_tac "(ifex_ite_opt (restrict_top i y False) (restrict_top t y False) (restrict_top e y False)) =
                         (ifex_ite (restrict_top i y False) (restrict_top t y False) (restrict_top e y False))")
       subgoal by force
       subgoal using restrict_top_ifex_minimal_invar restrict_top_ifex_ordered_invar by metis
       subgoal using restrict_top_ifex_minimal_invar restrict_top_ifex_ordered_invar by metis
      done
   done
done

lemma ro_ifexI: "(a,b) ∈ bf_ifex_rel ⟹ ro_ifex b" by (simp add: ifex_minimal_implied ifex_ordered_implied)

theorem ifex_ite_opt_rel_bf: "
  (fi,i) ∈ bf_ifex_rel ⟹
  (ft,t) ∈ bf_ifex_rel ⟹
  (fe,e) ∈ bf_ifex_rel ⟹
  ((bf_ite fi ft fe), (ifex_ite_opt i t e)) ∈ bf_ifex_rel"
using ifex_ite_rel_bf ifex_ite_opt_eq ro_ifexI by metis


lemma restrict_top_bf_ifex_rel:
"(f, i) ∈ bf_ifex_rel ⟹ ∃f'. (f', restrict_top i var val) ∈ bf_ifex_rel"
  unfolding bf_ifex_rel_def using restrict_top_ifex_minimal_invar restrict_top_ifex_ordered_invar
by fast

lemma param_opt_lowest_tops_lem: "param_opt i t e = None ⟹ ∃y. lowest_tops [i,t,e] = Some y"
  by (cases i) (auto simp add: param_opt_def)


fun ifex_sat where
"ifex_sat Trueif = Some (const False)" |
"ifex_sat Falseif = None" |
"ifex_sat (IF v t e) =
  (case ifex_sat e of 
    Some a ⇒ Some (a(v:=False)) |
    None ⇒ (case ifex_sat t of
      Some a ⇒ Some (a(v:=True)) |
      None ⇒ None))
"

lemma ifex_sat_untouched_False: "v ∉ ifex_var_set i ⟹ ifex_sat i = Some a ⟹ a v = False"
proof(induction i arbitrary: a)
  case (IF v1 t e)
  have ni: "v ∉ ifex_var_set t" "v ∉ ifex_var_set e" using IF.prems(1) by simp_all
  have ne: "v1 ≠ v" using IF.prems(1) by force
  show ?case proof(cases "ifex_sat e")
    case (Some as)
    with IF.prems(2) have au: "a = as(v1 := False)" by simp
    moreover from IF.IH(2)[OF ni(2)] have "as v = False" using Some .
    ultimately show ?thesis using ne by simp
  next
    case None
    obtain as where Some: "ifex_sat t = Some as" using None IF.prems(2) by fastforce
    with IF.prems(2) None have au: "a = as(v1 := True)" by(simp)
    moreover from IF.IH(1)[OF ni(1)] have "as v = False" using Some .
    ultimately show ?thesis using ne by simp
  qed (* this proof seems to complicated… *)
qed(simp_all add: fun_eq_iff)

lemma ifex_upd_other: "v ∉ ifex_var_set i ⟹ val_ifex i (a(v:=any)) = val_ifex i a" 
proof(induction i)
  case (IF v1 t e)
  have prems: "v ∉ ifex_var_set t " "v ∉ ifex_var_set e" using IF.prems by simp_all
  from IF.prems have ne: "v1 ≠ v" by clarsimp
  show ?case by(simp only: val_ifex.simps fun_upd_other[OF ne] ifex_vars.simps IF.IH(1)[OF prems(1)] IF.IH(2)[OF prems(2)] split: if_splits)
qed simp_all

(* something weaker than ifex_ordered *)
fun ifex_no_twice where
"ifex_no_twice (IF v t e) = (
  v ∉ (ifex_var_set t ∪ ifex_var_set e) ∧
  ifex_no_twice t ∧ ifex_no_twice e)" |
"ifex_no_twice _ = True"
lemma ordered_ifex_no_twiceI: "ifex_ordered i ⟹ ifex_no_twice i"
  by(induction i) (simp_all,blast)

lemma ifex_sat_NoneD: "ifex_sat i = None ⟹ val_ifex i ass = False"
  by(induction i) (simp_all split: option.splits)
lemma ifex_sat_SomeD: "ifex_no_twice i ⟹ ifex_sat i = Some ass ⟹ val_ifex i ass = True"
proof(induction i arbitrary: ass)
  case (IF v t e) 
  have ni: "v ∉ ifex_var_set t" "v ∉ ifex_var_set e" using IF.prems(1) by simp_all
  note IF.prems[unfolded ifex_sat.simps]
  thus ?case proof(cases "ifex_sat e")
    case (Some a) thus ?thesis using IF.prems 
      apply(clarsimp simp only: val_ifex.simps ifex_sat.simps option.simps fun_upd_same if_False ifex_upd_other[OF ni(2)])
      apply(rule IF.IH(2), simp_all)
      done
  next
    case None
    obtain a where Some: "ifex_sat t = Some a" using None IF.prems(2) by fastforce
    thus ?thesis using IF.prems
      by(clarsimp simp only: val_ifex.simps ifex_sat.simps option.simps fun_upd_same if_True None ifex_upd_other[OF ni(1)])
      (rule IF.IH(1), simp_all)
  qed
qed simp_all
lemma ifex_sat_NoneI: "ifex_no_twice i ⟹ (⋀ass. val_ifex i ass = False) ⟹ ifex_sat i = None" 
(* alternate proof: using ifex_sat_SomeD by fastforce *)
proof(rule ccontr, goal_cases)
  case 1
  from 1(3) obtain as where "ifex_sat i = Some as" by blast
  from ifex_sat_SomeD[OF 1(1) this] show False using 1(2) by simp
qed

fun ifex_sat_list where
"ifex_sat_list Trueif = Some []" |
"ifex_sat_list Falseif = None" |
"ifex_sat_list (IF v t e) =
  (case ifex_sat_list e of 
    Some a ⇒ Some ((v,False)#a) |
    None ⇒ (case ifex_sat_list t of
      Some a ⇒ Some ((v,True)#a) |
      None ⇒ None))
"

definition "update_assignment_alt u as = (λv. case map_of u v of None ⇒ as v | Some n ⇒ n)"
fun update_assignment where
"update_assignment ((v,u)#us) as = (update_assignment us as)(v:=u)" |
"update_assignment [] as = as"

lemma update_assignment_notin: "a ∉ fst ` set us ⟹ update_assignment us as a = as a"
by(induction us) clarsimp+

lemma update_assignment_alt: "update_assignment u as = update_assignment_alt u as"
by(induction u arbitrary: as) (clarsimp simp: update_assignment_alt_def fun_eq_iff)+

lemma update_assignment: "distinct (map fst ((v,u)#us)) ⟹ update_assignment ((v,u)#us) as = update_assignment us (as(v:=u))"
unfolding update_assignment_alt update_assignment_alt_def
unfolding fun_eq_iff
by(clarsimp split: option.splits) force 

lemma ass_upd_same: "update_assignment ((v, u) # a) ass v = u" by simp

lemma ifex_sat_list_subset:  "ifex_sat_list t = Some u ⟹ fst ` set u ⊆ ifex_var_set t" 
proof(induction t arbitrary: u)
  case (IF v t e)
  show ?case
  proof(cases "ifex_sat_list e")
    case (Some ue)
    note IF.IH(2)[OF this]
    hence "fst ` set ue ⊆ ifex_var_set (IF v t e)" by simp blast
    moreover have "fst ` set u = insert v (fst ` set ue)" using IF.prems Some by force 
    ultimately show ?thesis by simp
  next
    case None
    with IF.prems obtain ut where Some: "ifex_sat_list t = Some ut" by(simp split: option.splits)
    note IF.IH(1)[OF this]
    hence "fst ` set ut ⊆ ifex_var_set (IF v t e)" by simp blast
    moreover have "fst ` set u = insert v (fst ` set ut)" using IF.prems None Some by force 
    ultimately show ?thesis by simp
  qed
qed simp_all

lemma sat_list_distinct: "ifex_no_twice t ⟹ ifex_sat_list t = Some u ⟹ distinct (map fst u)"
proof(induction t arbitrary: u)
  case (IF v t e)
  from IF.prems have nt: "ifex_no_twice t" "ifex_no_twice e" by simp_all
  note mIH = IF.IH(1)[OF this(1)] IF.IH(2)[OF this(2)]
  show ?case
  proof(cases "ifex_sat_list e")
    case (Some a)
    note mIH = mIH(2)[OF this]
    thus ?thesis using IF.prems ifex_sat_list.simps Some ifex_sat_list_subset by fastforce
  next
    case None
    with IF.prems obtain ut where Some: "ifex_sat_list t = Some ut" by(simp split: option.splits)
    note mIH(1)[OF this]
    thus ?thesis using IF.prems ifex_sat_list.simps None Some ifex_sat_list_subset by fastforce
  qed
qed simp_all

lemma ifex_sat_list_NoneD: "ifex_sat_list i = None ⟹ val_ifex i ass = False"
  by(induction i) (simp_all split: option.splits)
lemma ifex_sat_list_SomeD: "ifex_no_twice i ⟹ ifex_sat_list i = Some u ⟹ ass = update_assignment u ass' ⟹ val_ifex i ass = True"
proof(induction i arbitrary: ass ass' u)
  case (IF v t e)
  have nt: "ifex_no_twice t" "ifex_no_twice e" using IF.prems(1) by simp_all
  have ni: "v ∉ ifex_var_set t" "v ∉ ifex_var_set e" using IF.prems(1) by simp_all
  note IF.prems[unfolded ifex_sat.simps]
  thus ?case proof(cases "ifex_sat_list e")
    case (Some a)
    have ef: "u = (v, False) # a" using IF.prems(2) Some by simp
    from IF.prems(3) have au: "ass = update_assignment a (ass'(v := False))" unfolding ef using update_assignment[OF sat_list_distinct[OF IF.prems(1,2), unfolded ef]] by presburger
    have avF: "ass v = False" using IF.prems(3)[symmetric] unfolding ef by clarsimp
    show ?thesis using IF.IH(2)[OF nt(2) Some au] Some IF.prems(2) avF by simp
  next
    case None
    obtain a where Some: "ifex_sat_list t = Some a" using None IF.prems(2) by fastforce
    have ef: "u = (v, True) # a" using IF.prems(2) None Some by simp
    from IF.prems(3) have au: "ass = update_assignment a (ass'(v := True))" unfolding ef using update_assignment[OF sat_list_distinct[OF IF.prems(1,2), unfolded ef]] by presburger
    have avT: "ass v = True" using IF.prems(3)[symmetric] unfolding ef by clarsimp
    show ?thesis using IF.IH(1)[OF nt(1) Some au] Some IF.prems(2) avT by simp
  qed
qed simp_all

fun sat_list_to_bdt where
"sat_list_to_bdt [] = Trueif" |
"sat_list_to_bdt ((v,u)#us) = (if u then IF v (sat_list_to_bdt us) Falseif else IF v Falseif (sat_list_to_bdt us))"

lemma "ifex_sat_list i = Some u ⟹ val_ifex (sat_list_to_bdt u) as ⟹ val_ifex i as"
proof(induction i arbitrary: u)
  case (IF v t e)
  show ?case proof(cases "ifex_sat_list e")
    case (Some a)
    note mIH = IF.IH(2)[OF this]
    have ef: "u = (v, False) # a" using IF.prems(1) Some by simp
    have avF: "as v = False" using IF.prems(2) unfolding ef by(simp split: if_splits)
    have "val_ifex (sat_list_to_bdt a) as" using IF.prems(2) unfolding ef using avF by simp
    note mIH = mIH[OF this]
    thus ?thesis using avF by simp
  next
    case None
    obtain a where Some: "ifex_sat_list t = Some a" using None IF.prems(1) by fastforce
    have ef: "u = (v, True) # a" using IF.prems(1) Some None by simp
    have avT: "as v = True" using IF.prems(2) unfolding ef by(simp split: if_splits)
    have "val_ifex (sat_list_to_bdt a) as" using IF.prems(2) unfolding ef using avT by simp
    note mIH = IF.IH(1)[OF Some this]
    thus ?thesis using avT by simp
  qed
qed simp_all

lemma bf_ifex_rel_consts[simp,intro!]:
  "(bf_True, Trueif) ∈ bf_ifex_rel"
  "(bf_False, Falseif) ∈ bf_ifex_rel"
by(fastforce simp add: bf_ifex_rel_def)+
lemma bf_ifex_rel_lit[simp,intro!]:
  "(bf_lit v, IFC v Trueif Falseif) ∈ bf_ifex_rel"
by(simp add: bf_ifex_rel_def IFC_def bf_lit_def)

lemma bf_ifex_rel_consts_ensured[simp]:
  "(bf_True,x) ∈ bf_ifex_rel ⟷ (x = Trueif)"
  "(bf_False,x) ∈ bf_ifex_rel ⟷ (x = Falseif)"
  by(auto simp add: bf_ifex_rel_def
             intro: roifex_Trueif_unique roifex_Falseif_unique)

(* reverse of the above *)
lemma bf_ifex_rel_consts_ensured_rev[simp]:
  "(x,Trueif) ∈ bf_ifex_rel ⟷ (x = bf_True)"
  "(x,Falseif) ∈ bf_ifex_rel ⟷ (x = bf_False)"
  by(simp_all add: bf_ifex_rel_def fun_eq_iff)

declare ifex_ite_opt.simps restrict_top.simps lowest_tops.simps[simp del]

end