Quelle  OrderArith.thy

(*  Title:      ZF/OrderArith.thy
  Author: Lawrence C Paulson, Cambridge University Computer Laboratory
  Copyright 1994 University of Cambridge
*)

section‹Combining Orderings: Foundations of Ordinal Arithmetic›

theory OrderArith imports Order Sum Ordinal begin

definition
  (*disjoint sum of two relations; underlies ordinal addition*)
  radd    :: "[i,i,i,i]==>i"  where
    "radd(A,r,B,s) ≡
                {z: (A+B) * (A+B).
                    (∃x y. z = ⟨Inl(x), Inr(y)⟩) |
                    (∃x' x. z = ⟨Inl(x'), Inl(x)⟩ ∧ ⟨x',x⟩:r) |
                    (∃y' y. z = ⟨Inr(y'), Inr(y)⟩ ∧ ⟨y',y⟩:s)}"

definition
  (*lexicographic product of two relations; underlies ordinal multiplication*)
  rmult   :: "[i,i,i,i]==>i"  where
    "rmult(A,r,B,s) ≡
                {z: (A*B) * (A*B).
                    ∃x' y' x y. z = ⟨⟨x',y'⟩, ⟨x,y⟩⟩ ∧
                       (⟨x',x⟩: r | (x'=x ∧ ⟨y',y⟩: s))}"

definition
  (*inverse image of a relation*)
  rvimage :: "[i,i,i]==>i"  where
    "rvimage(A,f,r) ≡ {z ∈ A*A. ∃x y. z = ⟨x,y⟩ ∧ ⟨f`x,f`y⟩: r}"

definition
  measure :: "[i, i==>i] ==> i"  where
    "measure(A,f) ≡ {⟨x,y⟩: A*A. f(x) < f(y)}"


subsection‹Addition of Relations -- Disjoint Sum›

subsubsection‹Rewrite rules. Can be used to obtain introduction rules›

lemma radd_Inl_Inr_iff [iff]:
    "⟨Inl(a), Inr(b)⟩ ∈ radd(A,r,B,s) ⟷ a ∈ A ∧ b ∈ B"
by (unfold radd_def, blast)

lemma radd_Inl_iff [iff]:
    "⟨Inl(a'), Inl(a)⟩ ∈ radd(A,r,B,s) ⟷ a':A ∧ a ∈ A ∧ ⟨a',a⟩:r"
by (unfold radd_def, blast)

lemma radd_Inr_iff [iff]:
    "⟨Inr(b'), Inr(b)⟩ ∈ radd(A,r,B,s) ⟷ b':B ∧ b ∈ B ∧ ⟨b',b⟩:s"
by (unfold radd_def, blast)

lemma radd_Inr_Inl_iff [simp]:
    "⟨Inr(b), Inl(a)⟩ ∈ radd(A,r,B,s) ⟷ False"
by (unfold radd_def, blast)

declare radd_Inr_Inl_iff [THEN iffD1, dest!]

subsubsection‹Elimination Rule›

lemma raddE:
    "[⟨p',p⟩ ∈ radd(A,r,B,s);
        ∧x y. [p'=Inl(x); x ∈ A; p=Inr(y); y ∈ B] ==> Q;
        ∧x' x. [p'=Inl(x'); p=Inl(x); ⟨x',x⟩: r; x':A; x ∈ A] ==> Q;
        ∧y' y. [p'=Inr(y'); p=Inr(y); ⟨y',y⟩: s; y':B; y ∈ B] ==> Q
\ ==> Q"
by (unfold radd_def, blast)

subsubsection‹Type checking›

lemma radd_type: "radd(A,r,B,s) ⊆ (A+B) * (A+B)"
  unfolding radd_def
apply (rule Collect_subset)
done

lemmas field_radd = radd_type [THEN field_rel_subset]

subsubsection‹Linearity›

lemma linear_radd:
    "[linear(A,r); linear(B,s)] ==> linear(A+B,radd(A,r,B,s))"
by (unfold linear_def, blast)


subsubsection‹Well-foundedness›

lemma wf_on_radd: "[wf[A](r); wf[B](s)] ==> wf[A+B](radd(A,r,B,s))"
apply (rule wf_onI2)
apply (subgoal_tac "∀x∈A. Inl (x) ∈ Ba")
 🍋 ‹Proving the lemma, which is needed twice!›
 prefer 2
 apply (erule_tac V = "y ∈ A + B" in thin_rl)
 apply (rule_tac ballI)
 apply (erule_tac r = r and a = x in wf_on_induct, assumption)
 apply blast
txt‹Returning to main part of proof›
apply safe
apply blast
apply (erule_tac r = s and a = ya in wf_on_induct, assumption, blast)
done

lemma wf_radd: "[wf(r); wf(s)] ==> wf(radd(field(r),r,field(s),s))"
apply (simp add: wf_iff_wf_on_field)
apply (rule wf_on_subset_A [OF _ field_radd])
apply (blast intro: wf_on_radd)
done

lemma well_ord_radd:
     "[well_ord(A,r); well_ord(B,s)] ==> well_ord(A+B, radd(A,r,B,s))"
apply (rule well_ordI)
apply (simp add: well_ord_def wf_on_radd)
apply (simp add: well_ord_def tot_ord_def linear_radd)
done

subsubsection‹An 🍋‹ord_iso› congruence law›

lemma sum_bij:
     "[f ∈ bij(A,C); g ∈ bij(B,D)]
      ==> (λz∈A+B. case(λx. Inl(f`x), λy. Inr(g`y), z)) ∈ bij(A+B, C+D)"
apply (rule_tac d = "case (λx. Inl (converse(f)`x), λy. Inr(converse(g)`y))"
       in lam_bijective)
apply (typecheck add: bij_is_inj inj_is_fun)
apply (auto simp add: left_inverse_bij right_inverse_bij)
done

lemma sum_ord_iso_cong:
    "[f ∈ ord_iso(A,r,A',r'); g ∈ ord_iso(B,s,B',s')] ==>
            (λz∈A+B. case(λx. Inl(f`x), λy. Inr(g`y), z))
            ∈ ord_iso(A+B, radd(A,r,B,s), A'+B', radd(A',r',B',s'))"
  unfolding ord_iso_def
apply (safe intro!: sum_bij)
(*Do the beta-reductions now*)
apply (auto cong add: conj_cong simp add: bij_is_fun [THEN apply_type])
done

(*Could we prove an ord_iso result?  Perhaps
     ord_iso(A+B, radd(A,r,B,s), A \<union> B, r \<union> s) *)
lemma sum_disjoint_bij: "A ∩ B = 0 ==>
            (λz∈A+B. case(λx. x, λy. y, z)) ∈ bij(A+B, A ∪ B)"
apply (rule_tac d = "λz. if z ∈ A then Inl (z) else Inr (z) " in lam_bijective)
apply auto
done

subsubsection‹Associativity›

lemma sum_assoc_bij:
     "(λz∈(A+B)+C. case(case(Inl, λy. Inr(Inl(y))), λy. Inr(Inr(y)), z))
      ∈ bij((A+B)+C, A+(B+C))"
apply (rule_tac d = "case (λx. Inl (Inl (x)), case (λx. Inl (Inr (x)), Inr))"
       in lam_bijective)
apply auto
done

lemma sum_assoc_ord_iso:
     "(λz∈(A+B)+C. case(case(Inl, λy. Inr(Inl(y))), λy. Inr(Inr(y)), z))
      ∈ ord_iso((A+B)+C, radd(A+B, radd(A,r,B,s), C, t),
                A+(B+C), radd(A, r, B+C, radd(B,s,C,t)))"
by (rule sum_assoc_bij [THEN ord_isoI], auto)


subsection‹Multiplication of Relations -- Lexicographic Product›

subsubsection‹Rewrite rule. Can be used to obtain introduction rules›

lemma  rmult_iff [iff]:
    "⟨⟨a',b'⟩, ⟨a,b⟩⟩ ∈ rmult(A,r,B,s) ⟷
            (⟨a',a⟩: r ∧ a':A ∧ a ∈ A ∧ b': B ∧ b ∈ B) |
            (⟨b',b⟩: s ∧ a'=a ∧ a ∈ A ∧ b': B ∧ b ∈ B)"

by (unfold rmult_def, blast)

lemma rmultE:
    "[⟨⟨a',b'⟩, ⟨a,b⟩⟩ ∈ rmult(A,r,B,s);
        [⟨a',a⟩: r; a':A; a ∈ A; b':B; b ∈ B] ==> Q;
        [⟨b',b⟩: s; a ∈ A; a'=a; b':B; b ∈ B] ==> Q
\ ==> Q"
by blast

subsubsection‹Type checking›

lemma rmult_type: "rmult(A,r,B,s) ⊆ (A*B) * (A*B)"
by (unfold rmult_def, rule Collect_subset)

lemmas field_rmult = rmult_type [THEN field_rel_subset]

subsubsection‹Linearity›

lemma linear_rmult:
    "[linear(A,r); linear(B,s)] ==> linear(A*B,rmult(A,r,B,s))"
by (simp add: linear_def, blast)

subsubsection‹Well-foundedness›

lemma wf_on_rmult: "[wf[A](r); wf[B](s)] ==> wf[A*B](rmult(A,r,B,s))"
apply (rule wf_onI2)
apply (erule SigmaE)
apply (erule ssubst)
apply (subgoal_tac "∀b∈B. ⟨x,b⟩: Ba", blast)
apply (erule_tac a = x in wf_on_induct, assumption)
apply (rule ballI)
apply (erule_tac a = b in wf_on_induct, assumption)
apply (best elim!: rmultE bspec [THEN mp])
done


lemma wf_rmult: "[wf(r); wf(s)] ==> wf(rmult(field(r),r,field(s),s))"
apply (simp add: wf_iff_wf_on_field)
apply (rule wf_on_subset_A [OF _ field_rmult])
apply (blast intro: wf_on_rmult)
done

lemma well_ord_rmult:
     "[well_ord(A,r); well_ord(B,s)] ==> well_ord(A*B, rmult(A,r,B,s))"
apply (rule well_ordI)
apply (simp add: well_ord_def wf_on_rmult)
apply (simp add: well_ord_def tot_ord_def linear_rmult)
done


subsubsection‹An 🍋‹ord_iso› congruence law›

lemma prod_bij:
     "[f ∈ bij(A,C); g ∈ bij(B,D)]
      ==> (lam ⟨x,y⟩:A*B. ⟨f`x, g`y⟩) ∈ bij(A*B, C*D)"
apply (rule_tac d = "λ⟨x,y⟩. ⟨converse (f) `x, converse (g) `y⟩"
       in lam_bijective)
apply (typecheck add: bij_is_inj inj_is_fun)
apply (auto simp add: left_inverse_bij right_inverse_bij)
done

lemma prod_ord_iso_cong:
    "[f ∈ ord_iso(A,r,A',r'); g ∈ ord_iso(B,s,B',s')]
     ==> (lam ⟨x,y⟩:A*B. ⟨f`x, g`y⟩)
         ∈ ord_iso(A*B, rmult(A,r,B,s), A'*B', rmult(A',r',B',s'))"
  unfolding ord_iso_def
apply (safe intro!: prod_bij)
apply (simp_all add: bij_is_fun [THEN apply_type])
apply (blast intro: bij_is_inj [THEN inj_apply_equality])
done

lemma singleton_prod_bij: "(λz∈A. ⟨x,z⟩) ∈ bij(A, {x}*A)"
by (rule_tac d = snd in lam_bijective, auto)

(*Used??*)
lemma singleton_prod_ord_iso:
     "well_ord({x},xr) ==>
          (λz∈A. ⟨x,z⟩) ∈ ord_iso(A, r, {x}*A, rmult({x}, xr, A, r))"
apply (rule singleton_prod_bij [THEN ord_isoI])
apply (simp (no_asm_simp))
apply (blast dest: well_ord_is_wf [THEN wf_on_not_refl])
done

(*Here we build a complicated function term, then simplify it using
  case_cong, id_conv, comp_lam, case_case.*)
lemma prod_sum_singleton_bij:
     "a∉C ==>
       (λx∈C*B + D. case(λx. x, λy.⟨a,y⟩, x))
       ∈ bij(C*B + D, C*B ∪ {a}*D)"
apply (rule subst_elem)
apply (rule id_bij [THEN sum_bij, THEN comp_bij])
apply (rule singleton_prod_bij)
apply (rule sum_disjoint_bij, blast)
apply (simp (no_asm_simp) cong add: case_cong)
apply (rule comp_lam [THEN trans, symmetric])
apply (fast elim!: case_type)
apply (simp (no_asm_simp) add: case_case)
done

lemma prod_sum_singleton_ord_iso:
 "[a ∈ A; well_ord(A,r)] ==>
    (λx∈pred(A,a,r)*B + pred(B,b,s). case(λx. x, λy.⟨a,y⟩, x))
    ∈ ord_iso(pred(A,a,r)*B + pred(B,b,s),
                  radd(A*B, rmult(A,r,B,s), B, s),
              pred(A,a,r)*B ∪ {a}*pred(B,b,s), rmult(A,r,B,s))"
apply (rule prod_sum_singleton_bij [THEN ord_isoI])
apply (simp (no_asm_simp) add: pred_iff well_ord_is_wf [THEN wf_on_not_refl])
apply (auto elim!: well_ord_is_wf [THEN wf_on_asym] predE)
done

subsubsection‹Distributive law›

lemma sum_prod_distrib_bij:
     "(lam ⟨x,z⟩:(A+B)*C. case(λy. Inl(⟨y,z⟩), λy. Inr(⟨y,z⟩), x))
      ∈ bij((A+B)*C, (A*C)+(B*C))"
by (rule_tac d = "case (λ⟨x,y⟩.⟨Inl (x),y⟩, λ⟨x,y⟩.⟨Inr (x),y⟩) "
    in lam_bijective, auto)

lemma sum_prod_distrib_ord_iso:
 "(lam ⟨x,z⟩:(A+B)*C. case(λy. Inl(⟨y,z⟩), λy. Inr(⟨y,z⟩), x))
  ∈ ord_iso((A+B)*C, rmult(A+B, radd(A,r,B,s), C, t),
            (A*C)+(B*C), radd(A*C, rmult(A,r,C,t), B*C, rmult(B,s,C,t)))"
by (rule sum_prod_distrib_bij [THEN ord_isoI], auto)

subsubsection‹Associativity›

lemma prod_assoc_bij:
     "(lam ⟨⟨x,y⟩, z⟩:(A*B)*C. ⟨x,⟨y,z⟩⟩) ∈ bij((A*B)*C, A*(B*C))"
by (rule_tac d = "λ⟨x, ⟨y,z⟩⟩. ⟨⟨x,y⟩, z⟩" in lam_bijective, auto)

lemma prod_assoc_ord_iso:
 "(lam ⟨⟨x,y⟩, z⟩:(A*B)*C. ⟨x,⟨y,z⟩⟩)
  ∈ ord_iso((A*B)*C, rmult(A*B, rmult(A,r,B,s), C, t),
            A*(B*C), rmult(A, r, B*C, rmult(B,s,C,t)))"
by (rule prod_assoc_bij [THEN ord_isoI], auto)

subsection‹Inverse Image of a Relation›

subsubsection‹Rewrite rule›

lemma rvimage_iff: "⟨a,b⟩ ∈ rvimage(A,f,r) ⟷ ⟨f`a,f`b⟩: r ∧ a ∈ A ∧ b ∈ A"
by (unfold rvimage_def, blast)

subsubsection‹Type checking›

lemma rvimage_type: "rvimage(A,f,r) ⊆ A*A"
by (unfold rvimage_def, rule Collect_subset)

lemmas field_rvimage = rvimage_type [THEN field_rel_subset]

lemma rvimage_converse: "rvimage(A,f, converse(r)) = converse(rvimage(A,f,r))"
by (unfold rvimage_def, blast)


subsubsection‹Partial Ordering Properties›

lemma irrefl_rvimage:
    "[f ∈ inj(A,B); irrefl(B,r)] ==> irrefl(A, rvimage(A,f,r))"
  unfolding irrefl_def rvimage_def
apply (blast intro: inj_is_fun [THEN apply_type])
done

lemma trans_on_rvimage:
    "[f ∈ inj(A,B); trans[B](r)] ==> trans[A](rvimage(A,f,r))"
  unfolding trans_on_def rvimage_def
apply (blast intro: inj_is_fun [THEN apply_type])
done

lemma part_ord_rvimage:
    "[f ∈ inj(A,B); part_ord(B,r)] ==> part_ord(A, rvimage(A,f,r))"
  unfolding part_ord_def
apply (blast intro!: irrefl_rvimage trans_on_rvimage)
done

subsubsection‹Linearity›

lemma linear_rvimage:
    "[f ∈ inj(A,B); linear(B,r)] ==> linear(A,rvimage(A,f,r))"
apply (simp add: inj_def linear_def rvimage_iff)
apply (blast intro: apply_funtype)
done

lemma tot_ord_rvimage:
    "[f ∈ inj(A,B); tot_ord(B,r)] ==> tot_ord(A, rvimage(A,f,r))"
  unfolding tot_ord_def
apply (blast intro!: part_ord_rvimage linear_rvimage)
done


subsubsection‹Well-foundedness›

lemma wf_rvimage [intro!]: "wf(r) ==> wf(rvimage(A,f,r))"
apply (simp (no_asm_use) add: rvimage_def wf_eq_minimal)
apply clarify
apply (subgoal_tac "∃w. w ∈ {w: {f`x. x ∈ Q}. ∃x. x ∈ Q ∧ (f`x = w) }")
 apply (erule allE)
 apply (erule impE)
 apply assumption
 apply blast
apply blast
done

text‹But note that the combination of ‹wf_imp_wf_on› and
  ‹wf_rvimage› gives 🍋‹wf(r) ==> wf[C](rvimage(A,f,r))›\›
lemma wf_on_rvimage: "[f ∈ A→B; wf[B](r)] ==> wf[A](rvimage(A,f,r))"
apply (rule wf_onI2)
apply (subgoal_tac "∀z∈A. f`z=f`y ⟶ z ∈ Ba")
 apply blast
apply (erule_tac a = "f`y" in wf_on_induct)
 apply (blast intro!: apply_funtype)
apply (blast intro!: apply_funtype dest!: rvimage_iff [THEN iffD1])
done

(*Note that we need only wf[A](...) and linear(A,...) to get the result!*)
lemma well_ord_rvimage:
     "[f ∈ inj(A,B); well_ord(B,r)] ==> well_ord(A, rvimage(A,f,r))"
apply (rule well_ordI)
  unfolding well_ord_def tot_ord_def
apply (blast intro!: wf_on_rvimage inj_is_fun)
apply (blast intro!: linear_rvimage)
done

lemma ord_iso_rvimage:
    "f ∈ bij(A,B) ==> f ∈ ord_iso(A, rvimage(A,f,s), B, s)"
  unfolding ord_iso_def
apply (simp add: rvimage_iff)
done

lemma ord_iso_rvimage_eq:
    "f ∈ ord_iso(A,r, B,s) ==> rvimage(A,f,s) = r ∩ A*A"
by (unfold ord_iso_def rvimage_def, blast)


subsection‹Every well-founded relation is a subset of some inverse image of
  an ordinal›

lemma wf_rvimage_Ord: "Ord(i) ==> wf(rvimage(A, f, Memrel(i)))"
by (blast intro: wf_rvimage wf_Memrel)


definition
  wfrank :: "[i,i]==>i"  where
    "wfrank(r,a) ≡ wfrec(r, a, λx f. ∪y ∈ r-``{x}. succ(f`y))"

definition
  wftype :: "i==>i"  where
    "wftype(r) ≡ ∪y ∈ range(r). succ(wfrank(r,y))"

lemma wfrank: "wf(r) ==> wfrank(r,a) = (∪y ∈ r-``{a}. succ(wfrank(r,y)))"
by (subst wfrank_def [THEN def_wfrec], simp_all)

lemma Ord_wfrank: "wf(r) ==> Ord(wfrank(r,a))"
apply (rule_tac a=a in wf_induct, assumption)
apply (subst wfrank, assumption)
apply (rule Ord_succ [THEN Ord_UN], blast)
done

lemma wfrank_lt: "[wf(r); ⟨a,b⟩ ∈ r] ==> wfrank(r,a) < wfrank(r,b)"
apply (rule_tac a1 = b in wfrank [THEN ssubst], assumption)
apply (rule UN_I [THEN ltI])
apply (simp add: Ord_wfrank vimage_iff)+
done

lemma Ord_wftype: "wf(r) ==> Ord(wftype(r))"
by (simp add: wftype_def Ord_wfrank)

lemma wftypeI: "[wf(r); x ∈ field(r)] ==> wfrank(r,x) ∈ wftype(r)"
apply (simp add: wftype_def)
apply (blast intro: wfrank_lt [THEN ltD])
done


lemma wf_imp_subset_rvimage:
     "[wf(r); r ⊆ A*A] ==> ∃i f. Ord(i) ∧ r ⊆ rvimage(A, f, Memrel(i))"
apply (rule_tac x="wftype(r)" in exI)
apply (rule_tac x="λx∈A. wfrank(r,x)" in exI)
apply (simp add: Ord_wftype, clarify)
apply (frule subsetD, assumption, clarify)
apply (simp add: rvimage_iff wfrank_lt [THEN ltD])
apply (blast intro: wftypeI)
done

theorem wf_iff_subset_rvimage:
  "relation(r) ==> wf(r) ⟷ (∃i f A. Ord(i) ∧ r ⊆ rvimage(A, f, Memrel(i)))"
by (blast dest!: relation_field_times_field wf_imp_subset_rvimage
          intro: wf_rvimage_Ord [THEN wf_subset])


subsection‹Other Results›

lemma wf_times: "A ∩ B = 0 ==> wf(A*B)"
by (simp add: wf_def, blast)

text‹Could also be used to prove ‹wf_radd›\ lemma wf_Un:
     "[range(r) ∩ domain(s) = 0; wf(r); wf(s)] ==> wf(r ∪ s)"
apply (simp add: wf_def, clarify)
apply (rule equalityI)
 prefer 2 apply blast
apply clarify
apply (drule_tac x=Z in spec)
apply (drule_tac x="Z ∩ domain(s)" in spec)
apply simp
apply (blast intro: elim: equalityE)
done

subsubsection‹The Empty Relation›

lemma wf0: "wf(0)"
by (simp add: wf_def, blast)

lemma linear0: "linear(0,0)"
by (simp add: linear_def)

lemma well_ord0: "well_ord(0,0)"
by (blast intro: wf_imp_wf_on well_ordI wf0 linear0)

subsubsection‹The "measure" relation is useful with wfrec›

lemma measure_eq_rvimage_Memrel:
     "measure(A,f) = rvimage(A,Lambda(A,f),Memrel(Collect(RepFun(A,f),Ord)))"
apply (simp (no_asm) add: measure_def rvimage_def Memrel_iff)
apply (rule equalityI, auto)
apply (auto intro: Ord_in_Ord simp add: lt_def)
done

lemma wf_measure [iff]: "wf(measure(A,f))"
by (simp (no_asm) add: measure_eq_rvimage_Memrel wf_Memrel wf_rvimage)

lemma measure_iff [iff]: "⟨x,y⟩ ∈ measure(A,f) ⟷ x ∈ A ∧ y ∈ A ∧ f(x)
by (simp (no_asm) add: measure_def)

lemma linear_measure:
 assumes Ordf: "∧x. x ∈ A ==> Ord(f(x))"
     and inj:  "∧x y. [x ∈ A; y ∈ A; f(x) = f(y)] ==> x=y"
 shows "linear(A, measure(A,f))"
apply (auto simp add: linear_def)
apply (rule_tac i="f(x)" and j="f(y)" in Ord_linear_lt)
    apply (simp_all add: Ordf)
apply (blast intro: inj)
done

lemma wf_on_measure: "wf[B](measure(A,f))"
by (rule wf_imp_wf_on [OF wf_measure])

lemma well_ord_measure:
 assumes Ordf: "∧x. x ∈ A ==> Ord(f(x))"
     and inj:  "∧x y. [x ∈ A; y ∈ A; f(x) = f(y)] ==> x=y"
 shows "well_ord(A, measure(A,f))"
apply (rule well_ordI)
apply (rule wf_on_measure)
apply (blast intro: linear_measure Ordf inj)
done

lemma measure_type: "measure(A,f) ⊆ A*A"
by (auto simp add: measure_def)

subsubsection‹Well-foundedness of Unions›

lemma wf_on_Union:
 assumes wfA: "wf[A](r)"
     and wfB: "∧a. a∈A ==> wf[B(a)](s)"
     and ok: "∧a u v. [⟨u,v⟩ ∈ s; v ∈ B(a); a ∈ A]
                       ==> (∃a'∈A. ⟨a',a⟩ ∈ r ∧ u ∈ B(a')) | u ∈ B(a)"
 shows "wf[∪a∈A. B(a)](s)"
apply (rule wf_onI2)
apply (erule UN_E)
apply (subgoal_tac "∀z ∈ B(a). z ∈ Ba", blast)
apply (rule_tac a = a in wf_on_induct [OF wfA], assumption)
apply (rule ballI)
apply (rule_tac a = z in wf_on_induct [OF wfB], assumption, assumption)
apply (rename_tac u)
apply (drule_tac x=u in bspec, blast)
apply (erule mp, clarify)
apply (frule ok, assumption+, blast)
done

subsubsection‹Bijections involving Powersets›

lemma Pow_sum_bij:
    "(λZ ∈ Pow(A+B). ⟨{x ∈ A. Inl(x) ∈ Z}, {y ∈ B. Inr(y) ∈ Z}⟩)
     ∈ bij(Pow(A+B), Pow(A)*Pow(B))"
apply (rule_tac d = "λ⟨X,Y⟩. {Inl (x). x ∈ X} ∪ {Inr (y). y ∈ Y}"
       in lam_bijective)
apply force+
done

text‹As a special case, we have 🍋‹bij(Pow(A*B), A → Pow(B))›\›
lemma Pow_Sigma_bij:
    "(λr ∈ Pow(Sigma(A,B)). λx ∈ A. r``{x})
     ∈ bij(Pow(Sigma(A,B)), ∏x ∈ A. Pow(B(x)))"
apply (rule_tac d = "λf. ∪x ∈ A. ∪y ∈ f`x. {⟨x,y⟩}" in lam_bijective)
apply (blast intro: lam_type)
apply (blast dest: apply_type, simp_all)
apply fast (*strange, but blast can't do it*)
apply (rule fun_extension, auto)
by blast

end