Theory FuncSet

(*  Title:      HOL/Library/FuncSet.thy
    Author:     Florian Kammueller and Lawrence C Paulson, Lukas Bulwahn
*)

section ‹Pi and Function Sets›

theory FuncSet
  imports Main
  abbrevs PiE = "Pi⇩E"
    and PIE = "Π⇩E"
begin

definition Pi :: "'a set ⇒ ('a ⇒ 'b set) ⇒ ('a ⇒ 'b) set"
  where "Pi A B = {f. ∀x. x ∈ A ⟶ f x ∈ B x}"

definition extensional :: "'a set ⇒ ('a ⇒ 'b) set"
  where "extensional A = {f. ∀x. x ∉ A ⟶ f x = undefined}"

definition "restrict" :: "('a ⇒ 'b) ⇒ 'a set ⇒ 'a ⇒ 'b"
  where "restrict f A = (λx. if x ∈ A then f x else undefined)"

abbreviation funcset :: "'a set ⇒ 'b set ⇒ ('a ⇒ 'b) set"  (infixr "→" 60)
  where "A → B ≡ Pi A (λ_. B)"

syntax
  "_Pi" :: "pttrn ⇒ 'a set ⇒ 'b set ⇒ ('a ⇒ 'b) set"  ("(3Π _∈_./ _)"   10)
  "_lam" :: "pttrn ⇒ 'a set ⇒ ('a ⇒ 'b) ⇒ ('a ⇒ 'b)"  ("(3λ_∈_./ _)" [0,0,3] 3)
translations
  "Π x∈A. B" ⇌ "CONST Pi A (λx. B)"
  "λx∈A. f" ⇌ "CONST restrict (λx. f) A"

definition "compose" :: "'a set ⇒ ('b ⇒ 'c) ⇒ ('a ⇒ 'b) ⇒ ('a ⇒ 'c)"
  where "compose A g f = (λx∈A. g (f x))"


subsection ‹Basic Properties of @{term Pi}›

lemma Pi_I[intro!]: "(⋀x. x ∈ A ⟹ f x ∈ B x) ⟹ f ∈ Pi A B"
  by (simp add: Pi_def)

lemma Pi_I'[simp]: "(⋀x. x ∈ A ⟶ f x ∈ B x) ⟹ f ∈ Pi A B"
  by (simp add:Pi_def)

lemma funcsetI: "(⋀x. x ∈ A ⟹ f x ∈ B) ⟹ f ∈ A → B"
  by (simp add: Pi_def)

lemma Pi_mem: "f ∈ Pi A B ⟹ x ∈ A ⟹ f x ∈ B x"
  by (simp add: Pi_def)

lemma Pi_iff: "f ∈ Pi I X ⟷ (∀i∈I. f i ∈ X i)"
  unfolding Pi_def by auto

lemma PiE [elim]: "f ∈ Pi A B ⟹ (f x ∈ B x ⟹ Q) ⟹ (x ∉ A ⟹ Q) ⟹ Q"
  by (auto simp: Pi_def)

lemma Pi_cong: "(⋀w. w ∈ A ⟹ f w = g w) ⟹ f ∈ Pi A B ⟷ g ∈ Pi A B"
  by (auto simp: Pi_def)

lemma funcset_id [simp]: "(λx. x) ∈ A → A"
  by auto

lemma funcset_mem: "f ∈ A → B ⟹ x ∈ A ⟹ f x ∈ B"
  by (simp add: Pi_def)

lemma funcset_image: "f ∈ A → B ⟹ f ` A ⊆ B"
  by auto

lemma image_subset_iff_funcset: "F ` A ⊆ B ⟷ F ∈ A → B"
  by auto

lemma Pi_eq_empty[simp]: "(Π x ∈ A. B x) = {} ⟷ (∃x∈A. B x = {})"
  apply (simp add: Pi_def)
  apply auto
  txt ‹Converse direction requires Axiom of Choice to exhibit a function
  picking an element from each non-empty @{term "B x"}›
  apply (drule_tac x = "λu. SOME y. y ∈ B u" in spec)
  apply auto
  apply (cut_tac P = "λy. y ∈ B x" in some_eq_ex)
  apply auto
  done

lemma Pi_empty [simp]: "Pi {} B = UNIV"
  by (simp add: Pi_def)

lemma Pi_Int: "Pi I E ∩ Pi I F = (Π i∈I. E i ∩ F i)"
  by auto

lemma Pi_UN:
  fixes A :: "nat ⇒ 'i ⇒ 'a set"
  assumes "finite I"
    and mono: "⋀i n m. i ∈ I ⟹ n ≤ m ⟹ A n i ⊆ A m i"
  shows "(⋃n. Pi I (A n)) = (Π i∈I. ⋃n. A n i)"
proof (intro set_eqI iffI)
  fix f
  assume "f ∈ (Π i∈I. ⋃n. A n i)"
  then have "∀i∈I. ∃n. f i ∈ A n i"
    by auto
  from bchoice[OF this] obtain n where n: "f i ∈ A (n i) i" if "i ∈ I" for i
    by auto
  obtain k where k: "n i ≤ k" if "i ∈ I" for i
    using ‹finite I› finite_nat_set_iff_bounded_le[of "n`I"] by auto
  have "f ∈ Pi I (A k)"
  proof (intro Pi_I)
    fix i
    assume "i ∈ I"
    from mono[OF this, of "n i" k] k[OF this] n[OF this]
    show "f i ∈ A k i" by auto
  qed
  then show "f ∈ (⋃n. Pi I (A n))"
    by auto
qed auto

lemma Pi_UNIV [simp]: "A → UNIV = UNIV"
  by (simp add: Pi_def)

text ‹Covariance of Pi-sets in their second argument›
lemma Pi_mono: "(⋀x. x ∈ A ⟹ B x ⊆ C x) ⟹ Pi A B ⊆ Pi A C"
  by auto

text ‹Contravariance of Pi-sets in their first argument›
lemma Pi_anti_mono: "A' ⊆ A ⟹ Pi A B ⊆ Pi A' B"
  by auto

lemma prod_final:
  assumes 1: "fst ∘ f ∈ Pi A B"
    and 2: "snd ∘ f ∈ Pi A C"
  shows "f ∈ (Π z ∈ A. B z × C z)"
proof (rule Pi_I)
  fix z
  assume z: "z ∈ A"
  have "f z = (fst (f z), snd (f z))"
    by simp
  also have "… ∈ B z × C z"
    by (metis SigmaI PiE o_apply 1 2 z)
  finally show "f z ∈ B z × C z" .
qed

lemma Pi_split_domain[simp]: "x ∈ Pi (I ∪ J) X ⟷ x ∈ Pi I X ∧ x ∈ Pi J X"
  by (auto simp: Pi_def)

lemma Pi_split_insert_domain[simp]: "x ∈ Pi (insert i I) X ⟷ x ∈ Pi I X ∧ x i ∈ X i"
  by (auto simp: Pi_def)

lemma Pi_cancel_fupd_range[simp]: "i ∉ I ⟹ x ∈ Pi I (B(i := b)) ⟷ x ∈ Pi I B"
  by (auto simp: Pi_def)

lemma Pi_cancel_fupd[simp]: "i ∉ I ⟹ x(i := a) ∈ Pi I B ⟷ x ∈ Pi I B"
  by (auto simp: Pi_def)

lemma Pi_fupd_iff: "i ∈ I ⟹ f ∈ Pi I (B(i := A)) ⟷ f ∈ Pi (I - {i}) B ∧ f i ∈ A"
  apply auto
  apply (drule_tac x=x in Pi_mem)
  apply (simp_all split: if_split_asm)
  apply (drule_tac x=i in Pi_mem)
  apply (auto dest!: Pi_mem)
  done


subsection ‹Composition With a Restricted Domain: @{term compose}›

lemma funcset_compose: "f ∈ A → B ⟹ g ∈ B → C ⟹ compose A g f ∈ A → C"
  by (simp add: Pi_def compose_def restrict_def)

lemma compose_assoc:
  assumes "f ∈ A → B"
    and "g ∈ B → C"
    and "h ∈ C → D"
  shows "compose A h (compose A g f) = compose A (compose B h g) f"
  using assms by (simp add: fun_eq_iff Pi_def compose_def restrict_def)

lemma compose_eq: "x ∈ A ⟹ compose A g f x = g (f x)"
  by (simp add: compose_def restrict_def)

lemma surj_compose: "f ` A = B ⟹ g ` B = C ⟹ compose A g f ` A = C"
  by (auto simp add: image_def compose_eq)


subsection ‹Bounded Abstraction: @{term restrict}›

lemma restrict_cong: "I = J ⟹ (⋀i. i ∈ J =simp=> f i = g i) ⟹ restrict f I = restrict g J"
  by (auto simp: restrict_def fun_eq_iff simp_implies_def)

lemma restrict_in_funcset: "(⋀x. x ∈ A ⟹ f x ∈ B) ⟹ (λx∈A. f x) ∈ A → B"
  by (simp add: Pi_def restrict_def)

lemma restrictI[intro!]: "(⋀x. x ∈ A ⟹ f x ∈ B x) ⟹ (λx∈A. f x) ∈ Pi A B"
  by (simp add: Pi_def restrict_def)

lemma restrict_apply[simp]: "(λy∈A. f y) x = (if x ∈ A then f x else undefined)"
  by (simp add: restrict_def)

lemma restrict_apply': "x ∈ A ⟹ (λy∈A. f y) x = f x"
  by simp

lemma restrict_ext: "(⋀x. x ∈ A ⟹ f x = g x) ⟹ (λx∈A. f x) = (λx∈A. g x)"
  by (simp add: fun_eq_iff Pi_def restrict_def)

lemma restrict_UNIV: "restrict f UNIV = f"
  by (simp add: restrict_def)

lemma inj_on_restrict_eq [simp]: "inj_on (restrict f A) A = inj_on f A"
  by (simp add: inj_on_def restrict_def)

lemma Id_compose: "f ∈ A → B ⟹ f ∈ extensional A ⟹ compose A (λy∈B. y) f = f"
  by (auto simp add: fun_eq_iff compose_def extensional_def Pi_def)

lemma compose_Id: "g ∈ A → B ⟹ g ∈ extensional A ⟹ compose A g (λx∈A. x) = g"
  by (auto simp add: fun_eq_iff compose_def extensional_def Pi_def)

lemma image_restrict_eq [simp]: "(restrict f A) ` A = f ` A"
  by (auto simp add: restrict_def)

lemma restrict_restrict[simp]: "restrict (restrict f A) B = restrict f (A ∩ B)"
  unfolding restrict_def by (simp add: fun_eq_iff)

lemma restrict_fupd[simp]: "i ∉ I ⟹ restrict (f (i := x)) I = restrict f I"
  by (auto simp: restrict_def)

lemma restrict_upd[simp]: "i ∉ I ⟹ (restrict f I)(i := y) = restrict (f(i := y)) (insert i I)"
  by (auto simp: fun_eq_iff)

lemma restrict_Pi_cancel: "restrict x I ∈ Pi I A ⟷ x ∈ Pi I A"
  by (auto simp: restrict_def Pi_def)


subsection ‹Bijections Between Sets›

text ‹The definition of @{const bij_betw} is in ‹Fun.thy›, but most of
the theorems belong here, or need at least @{term Hilbert_Choice}.›

lemma bij_betwI:
  assumes "f ∈ A → B"
    and "g ∈ B → A"
    and g_f: "⋀x. x∈A ⟹ g (f x) = x"
    and f_g: "⋀y. y∈B ⟹ f (g y) = y"
  shows "bij_betw f A B"
  unfolding bij_betw_def
proof
  show "inj_on f A"
    by (metis g_f inj_on_def)
  have "f ` A ⊆ B"
    using ‹f ∈ A → B› by auto
  moreover
  have "B ⊆ f ` A"
    by auto (metis Pi_mem ‹g ∈ B → A› f_g image_iff)
  ultimately show "f ` A = B"
    by blast
qed

lemma bij_betw_imp_funcset: "bij_betw f A B ⟹ f ∈ A → B"
  by (auto simp add: bij_betw_def)

lemma inj_on_compose: "bij_betw f A B ⟹ inj_on g B ⟹ inj_on (compose A g f) A"
  by (auto simp add: bij_betw_def inj_on_def compose_eq)

lemma bij_betw_compose: "bij_betw f A B ⟹ bij_betw g B C ⟹ bij_betw (compose A g f) A C"
  apply (simp add: bij_betw_def compose_eq inj_on_compose)
  apply (auto simp add: compose_def image_def)
  done

lemma bij_betw_restrict_eq [simp]: "bij_betw (restrict f A) A B = bij_betw f A B"
  by (simp add: bij_betw_def)


subsection ‹Extensionality›

lemma extensional_empty[simp]: "extensional {} = {λx. undefined}"
  unfolding extensional_def by auto

lemma extensional_arb: "f ∈ extensional A ⟹ x ∉ A ⟹ f x = undefined"
  by (simp add: extensional_def)

lemma restrict_extensional [simp]: "restrict f A ∈ extensional A"
  by (simp add: restrict_def extensional_def)

lemma compose_extensional [simp]: "compose A f g ∈ extensional A"
  by (simp add: compose_def)

lemma extensionalityI:
  assumes "f ∈ extensional A"
    and "g ∈ extensional A"
    and "⋀x. x ∈ A ⟹ f x = g x"
  shows "f = g"
  using assms by (force simp add: fun_eq_iff extensional_def)

lemma extensional_restrict:  "f ∈ extensional A ⟹ restrict f A = f"
  by (rule extensionalityI[OF restrict_extensional]) auto

lemma extensional_subset: "f ∈ extensional A ⟹ A ⊆ B ⟹ f ∈ extensional B"
  unfolding extensional_def by auto

lemma inv_into_funcset: "f ` A = B ⟹ (λx∈B. inv_into A f x) ∈ B → A"
  by (unfold inv_into_def) (fast intro: someI2)

lemma compose_inv_into_id: "bij_betw f A B ⟹ compose A (λy∈B. inv_into A f y) f = (λx∈A. x)"
  apply (simp add: bij_betw_def compose_def)
  apply (rule restrict_ext, auto)
  done

lemma compose_id_inv_into: "f ` A = B ⟹ compose B f (λy∈B. inv_into A f y) = (λx∈B. x)"
  apply (simp add: compose_def)
  apply (rule restrict_ext)
  apply (simp add: f_inv_into_f)
  done

lemma extensional_insert[intro, simp]:
  assumes "a ∈ extensional (insert i I)"
  shows "a(i := b) ∈ extensional (insert i I)"
  using assms unfolding extensional_def by auto

lemma extensional_Int[simp]: "extensional I ∩ extensional I' = extensional (I ∩ I')"
  unfolding extensional_def by auto

lemma extensional_UNIV[simp]: "extensional UNIV = UNIV"
  by (auto simp: extensional_def)

lemma restrict_extensional_sub[intro]: "A ⊆ B ⟹ restrict f A ∈ extensional B"
  unfolding restrict_def extensional_def by auto

lemma extensional_insert_undefined[intro, simp]:
  "a ∈ extensional (insert i I) ⟹ a(i := undefined) ∈ extensional I"
  unfolding extensional_def by auto

lemma extensional_insert_cancel[intro, simp]:
  "a ∈ extensional I ⟹ a ∈ extensional (insert i I)"
  unfolding extensional_def by auto


subsection ‹Cardinality›

lemma card_inj: "f ∈ A → B ⟹ inj_on f A ⟹ finite B ⟹ card A ≤ card B"
  by (rule card_inj_on_le) auto

lemma card_bij:
  assumes "f ∈ A → B" "inj_on f A"
    and "g ∈ B → A" "inj_on g B"
    and "finite A" "finite B"
  shows "card A = card B"
  using assms by (blast intro: card_inj order_antisym)


subsection ‹Extensional Function Spaces›

definition PiE :: "'a set ⇒ ('a ⇒ 'b set) ⇒ ('a ⇒ 'b) set"
  where "PiE S T = Pi S T ∩ extensional S"

abbreviation "Pi⇩E A B ≡ PiE A B"

syntax
  "_PiE" :: "pttrn ⇒ 'a set ⇒ 'b set ⇒ ('a ⇒ 'b) set"  ("(3Π⇩E _∈_./ _)" 10)
translations
  "Π⇩E x∈A. B" ⇌ "CONST Pi⇩E A (λx. B)"

abbreviation extensional_funcset :: "'a set ⇒ 'b set ⇒ ('a ⇒ 'b) set" (infixr "→⇩E" 60)
  where "A →⇩E B ≡ (Π⇩E i∈A. B)"

lemma extensional_funcset_def: "extensional_funcset S T = (S → T) ∩ extensional S"
  by (simp add: PiE_def)

lemma PiE_empty_domain[simp]: "Pi⇩E {} T = {λx. undefined}"
  unfolding PiE_def by simp

lemma PiE_UNIV_domain: "Pi⇩E UNIV T = Pi UNIV T"
  unfolding PiE_def by simp

lemma PiE_empty_range[simp]: "i ∈ I ⟹ F i = {} ⟹ (Π⇩E i∈I. F i) = {}"
  unfolding PiE_def by auto

lemma PiE_eq_empty_iff: "Pi⇩E I F = {} ⟷ (∃i∈I. F i = {})"
proof
  assume "Pi⇩E I F = {}"
  show "∃i∈I. F i = {}"
  proof (rule ccontr)
    assume "¬ ?thesis"
    then have "∀i. ∃y. (i ∈ I ⟶ y ∈ F i) ∧ (i ∉ I ⟶ y = undefined)"
      by auto
    from choice[OF this]
    obtain f where " ∀x. (x ∈ I ⟶ f x ∈ F x) ∧ (x ∉ I ⟶ f x = undefined)" ..
    then have "f ∈ Pi⇩E I F"
      by (auto simp: extensional_def PiE_def)
    with ‹Pi⇩E I F = {}› show False
      by auto
  qed
qed (auto simp: PiE_def)

lemma PiE_arb: "f ∈ Pi⇩E S T ⟹ x ∉ S ⟹ f x = undefined"
  unfolding PiE_def by auto (auto dest!: extensional_arb)

lemma PiE_mem: "f ∈ Pi⇩E S T ⟹ x ∈ S ⟹ f x ∈ T x"
  unfolding PiE_def by auto

lemma PiE_fun_upd: "y ∈ T x ⟹ f ∈ Pi⇩E S T ⟹ f(x := y) ∈ Pi⇩E (insert x S) T"
  unfolding PiE_def extensional_def by auto

lemma fun_upd_in_PiE: "x ∉ S ⟹ f ∈ Pi⇩E (insert x S) T ⟹ f(x := undefined) ∈ Pi⇩E S T"
  unfolding PiE_def extensional_def by auto

lemma PiE_insert_eq: "Pi⇩E (insert x S) T = (λ(y, g). g(x := y)) ` (T x × Pi⇩E S T)"
proof -
  {
    fix f assume "f ∈ Pi⇩E (insert x S) T" "x ∉ S"
    then have "f ∈ (λ(y, g). g(x := y)) ` (T x × Pi⇩E S T)"
      by (auto intro!: image_eqI[where x="(f x, f(x := undefined))"] intro: fun_upd_in_PiE PiE_mem)
  }
  moreover
  {
    fix f assume "f ∈ Pi⇩E (insert x S) T" "x ∈ S"
    then have "f ∈ (λ(y, g). g(x := y)) ` (T x × Pi⇩E S T)"
      by (auto intro!: image_eqI[where x="(f x, f)"] intro: fun_upd_in_PiE PiE_mem simp: insert_absorb)
  }
  ultimately show ?thesis
    by (auto intro: PiE_fun_upd)
qed

lemma PiE_Int: "Pi⇩E I A ∩ Pi⇩E I B = Pi⇩E I (λx. A x ∩ B x)"
  by (auto simp: PiE_def)

lemma PiE_cong: "(⋀i. i∈I ⟹ A i = B i) ⟹ Pi⇩E I A = Pi⇩E I B"
  unfolding PiE_def by (auto simp: Pi_cong)

lemma PiE_E [elim]:
  assumes "f ∈ Pi⇩E A B"
  obtains "x ∈ A" and "f x ∈ B x"
    | "x ∉ A" and "f x = undefined"
  using assms by (auto simp: Pi_def PiE_def extensional_def)

lemma PiE_I[intro!]:
  "(⋀x. x ∈ A ⟹ f x ∈ B x) ⟹ (⋀x. x ∉ A ⟹ f x = undefined) ⟹ f ∈ Pi⇩E A B"
  by (simp add: PiE_def extensional_def)

lemma PiE_mono: "(⋀x. x ∈ A ⟹ B x ⊆ C x) ⟹ Pi⇩E A B ⊆ Pi⇩E A C"
  by auto

lemma PiE_iff: "f ∈ Pi⇩E I X ⟷ (∀i∈I. f i ∈ X i) ∧ f ∈ extensional I"
  by (simp add: PiE_def Pi_iff)

lemma PiE_restrict[simp]:  "f ∈ Pi⇩E A B ⟹ restrict f A = f"
  by (simp add: extensional_restrict PiE_def)

lemma restrict_PiE[simp]: "restrict f I ∈ Pi⇩E I S ⟷ f ∈ Pi I S"
  by (auto simp: PiE_iff)

lemma PiE_eq_subset:
  assumes ne: "⋀i. i ∈ I ⟹ F i ≠ {}" "⋀i. i ∈ I ⟹ F' i ≠ {}"
    and eq: "Pi⇩E I F = Pi⇩E I F'"
    and "i ∈ I"
  shows "F i ⊆ F' i"
proof
  fix x
  assume "x ∈ F i"
  with ne have "∀j. ∃y. (j ∈ I ⟶ y ∈ F j ∧ (i = j ⟶ x = y)) ∧ (j ∉ I ⟶ y = undefined)"
    by auto
  from choice[OF this] obtain f
    where f: " ∀j. (j ∈ I ⟶ f j ∈ F j ∧ (i = j ⟶ x = f j)) ∧ (j ∉ I ⟶ f j = undefined)" ..
  then have "f ∈ Pi⇩E I F"
    by (auto simp: extensional_def PiE_def)
  then have "f ∈ Pi⇩E I F'"
    using assms by simp
  then show "x ∈ F' i"
    using f ‹i ∈ I› by (auto simp: PiE_def)
qed

lemma PiE_eq_iff_not_empty:
  assumes ne: "⋀i. i ∈ I ⟹ F i ≠ {}" "⋀i. i ∈ I ⟹ F' i ≠ {}"
  shows "Pi⇩E I F = Pi⇩E I F' ⟷ (∀i∈I. F i = F' i)"
proof (intro iffI ballI)
  fix i
  assume eq: "Pi⇩E I F = Pi⇩E I F'"
  assume i: "i ∈ I"
  show "F i = F' i"
    using PiE_eq_subset[of I F F', OF ne eq i]
    using PiE_eq_subset[of I F' F, OF ne(2,1) eq[symmetric] i]
    by auto
qed (auto simp: PiE_def)

lemma PiE_eq_iff:
  "Pi⇩E I F = Pi⇩E I F' ⟷ (∀i∈I. F i = F' i) ∨ ((∃i∈I. F i = {}) ∧ (∃i∈I. F' i = {}))"
proof (intro iffI disjCI)
  assume eq[simp]: "Pi⇩E I F = Pi⇩E I F'"
  assume "¬ ((∃i∈I. F i = {}) ∧ (∃i∈I. F' i = {}))"
  then have "(∀i∈I. F i ≠ {}) ∧ (∀i∈I. F' i ≠ {})"
    using PiE_eq_empty_iff[of I F] PiE_eq_empty_iff[of I F'] by auto
  with PiE_eq_iff_not_empty[of I F F'] show "∀i∈I. F i = F' i"
    by auto
next
  assume "(∀i∈I. F i = F' i) ∨ (∃i∈I. F i = {}) ∧ (∃i∈I. F' i = {})"
  then show "Pi⇩E I F = Pi⇩E I F'"
    using PiE_eq_empty_iff[of I F] PiE_eq_empty_iff[of I F'] by (auto simp: PiE_def)
qed

lemma extensional_funcset_fun_upd_restricts_rangeI:
  "∀y ∈ S. f x ≠ f y ⟹ f ∈ (insert x S) →⇩E T ⟹ f(x := undefined) ∈ S →⇩E (T - {f x})"
  unfolding extensional_funcset_def extensional_def
  apply auto
  apply (case_tac "x = xa")
  apply auto
  done

lemma extensional_funcset_fun_upd_extends_rangeI:
  assumes "a ∈ T" "f ∈ S →⇩E (T - {a})"
  shows "f(x := a) ∈ insert x S →⇩E  T"
  using assms unfolding extensional_funcset_def extensional_def by auto


subsubsection ‹Injective Extensional Function Spaces›

lemma extensional_funcset_fun_upd_inj_onI:
  assumes "f ∈ S →⇩E (T - {a})"
    and "inj_on f S"
  shows "inj_on (f(x := a)) S"
  using assms
  unfolding extensional_funcset_def by (auto intro!: inj_on_fun_updI)

lemma extensional_funcset_extend_domain_inj_on_eq:
  assumes "x ∉ S"
  shows "{f. f ∈ (insert x S) →⇩E T ∧ inj_on f (insert x S)} =
    (λ(y, g). g(x:=y)) ` {(y, g). y ∈ T ∧ g ∈ S →⇩E (T - {y}) ∧ inj_on g S}"
  using assms
  apply (auto del: PiE_I PiE_E)
  apply (auto intro: extensional_funcset_fun_upd_inj_onI
    extensional_funcset_fun_upd_extends_rangeI del: PiE_I PiE_E)
  apply (auto simp add: image_iff inj_on_def)
  apply (rule_tac x="xa x" in exI)
  apply (auto intro: PiE_mem del: PiE_I PiE_E)
  apply (rule_tac x="xa(x := undefined)" in exI)
  apply (auto intro!: extensional_funcset_fun_upd_restricts_rangeI)
  apply (auto dest!: PiE_mem split: if_split_asm)
  done

lemma extensional_funcset_extend_domain_inj_onI:
  assumes "x ∉ S"
  shows "inj_on (λ(y, g). g(x := y)) {(y, g). y ∈ T ∧ g ∈ S →⇩E (T - {y}) ∧ inj_on g S}"
  using assms
  apply (auto intro!: inj_onI)
  apply (metis fun_upd_same)
  apply (metis assms PiE_arb fun_upd_triv fun_upd_upd)
  done


subsubsection ‹Cardinality›

lemma finite_PiE: "finite S ⟹ (⋀i. i ∈ S ⟹ finite (T i)) ⟹ finite (Π⇩E i ∈ S. T i)"
  by (induct S arbitrary: T rule: finite_induct) (simp_all add: PiE_insert_eq)

lemma inj_combinator: "x ∉ S ⟹ inj_on (λ(y, g). g(x := y)) (T x × Pi⇩E S T)"
proof (safe intro!: inj_onI ext)
  fix f y g z
  assume "x ∉ S"
  assume fg: "f ∈ Pi⇩E S T" "g ∈ Pi⇩E S T"
  assume "f(x := y) = g(x := z)"
  then have *: "⋀i. (f(x := y)) i = (g(x := z)) i"
    unfolding fun_eq_iff by auto
  from this[of x] show "y = z" by simp
  fix i from *[of i] ‹x ∉ S› fg show "f i = g i"
    by (auto split: if_split_asm simp: PiE_def extensional_def)
qed

lemma card_PiE: "finite S ⟹ card (Π⇩E i ∈ S. T i) = (∏ i∈S. card (T i))"
proof (induct rule: finite_induct)
  case empty
  then show ?case by auto
next
  case (insert x S)
  then show ?case
    by (simp add: PiE_insert_eq inj_combinator card_image card_cartesian_product)
qed

end