import Mathlib
import Analysis.Section_3_1
import Analysis.Tags.Section_3_1

namespace Chapter3

-- <CONTEXT>
universe u v

class SetTheory where
  Set : Type u
  Object : Type v
  set_to_object : Set ↪ Object
  mem : Object → Set → Prop
  extensionality X Y : (∀ x, mem x X ↔ mem x Y) → X = Y
  emptyset: Set
  emptyset_mem x : ¬ mem x emptyset
  singleton : Object → Set
  singleton_axiom x y : mem x (singleton y) ↔ x = y
  union_pair : Set → Set → Set
  union_pair_axiom X Y x : mem x (union_pair X Y) ↔ (mem x X ∨ mem x Y)
  specify A (P: Subtype (mem . A) → Prop) : Set
  specification_axiom A (P: Subtype (mem . A) → Prop) :
    (∀ x, mem x (specify A P) → mem x A) ∧ ∀ x, mem x.val (specify A P) ↔ P x
  replace A (P: Subtype (mem . A) → Object → Prop)
    (hP: ∀ x y y', P x y ∧ P x y' → y = y') : Set
  replacement_axiom A (P: Subtype (mem . A) → Object → Prop)
    (hP: ∀ x y y', P x y ∧ P x y' → y = y') : ∀ y, mem y (replace A P hP) ↔ ∃ x, P x y
  nat : Set
  nat_equiv : ℕ ≃ Subtype (mem . nat)
  regularity_axiom A (hA : ∃ x, mem x A) :
    ∃ x, mem x A ∧ ∀ S, x = set_to_object S → ¬ ∃ y, mem y A ∧ mem y S
  pow : Set → Set → Set
  function_to_object (X: Set) (Y: Set) :
    (Subtype (mem . X) → Subtype (mem . Y)) ↪ Object
  powerset_axiom (X: Set) (Y: Set) (F:Object) :
    mem F (pow X Y) ↔ ∃ f: Subtype (mem . Y) → Subtype (mem . X),
    function_to_object Y X f = F
  union : Set → Set
  union_axiom A x : mem x (union A) ↔ ∃ S, mem x S ∧ mem (set_to_object S) A

export SetTheory (Set Object nat)

variable [SetTheory]

instance SetTheory.objects_mem_sets : Membership Object Set where
  mem X x := SetTheory.mem x X

instance SetTheory.sets_are_objects : Coe Set Object where
  coe X := SetTheory.set_to_object X

theorem SetTheory.Set.coe_eq {X Y:Set} (h: (X: Object) = (Y: Object)) : X = Y := by
  sorry

@[simp]
theorem SetTheory.Set.coe_eq_iff (X Y:Set) : (X: Object) = (Y: Object) ↔  X = Y := by
  sorry

@[ext]
theorem SetTheory.Set.ext {X Y:Set} (h: ∀ x, x ∈ X ↔ x ∈ Y) : X = Y := by
  sorry

instance SetTheory.Set.instEmpty : EmptyCollection Set where
  emptyCollection := SetTheory.emptyset

open SetTheory.Set

@[simp]
theorem SetTheory.Set.not_mem_empty : ∀ x, x ∉ (∅:Set) := by
  sorry

theorem SetTheory.Set.eq_empty_iff_forall_notMem {X:Set} : X = ∅ ↔ (∀ x, x ∉ X) := by
  sorry

theorem SetTheory.Set.empty_unique : ∃! (X:Set), ∀ x, x ∉ X := by
  sorry

lemma SetTheory.Set.nonempty_def {X:Set} (h: X ≠ ∅) : ∃ x, x ∈ X := by
  sorry

theorem SetTheory.Set.nonempty_of_inhabited {X:Set} {x:Object} (h:x ∈ X) : X ≠ ∅ := by
  sorry

instance SetTheory.Set.instSingleton : Singleton Object Set where
  singleton := SetTheory.singleton

@[simp]
theorem SetTheory.Set.mem_singleton (x a:Object) : x ∈ ({a}:Set) ↔ x = a := by
  sorry

instance SetTheory.Set.instUnion : Union Set where
  union := SetTheory.union_pair

@[simp]
theorem SetTheory.Set.mem_union (x:Object) (X Y:Set) : x ∈ (X ∪ Y) ↔ (x ∈ X ∨ x ∈ Y) := by
  sorry

instance SetTheory.Set.instInsert : Insert Object Set where
  insert x X := {x} ∪ X

@[simp]
theorem SetTheory.Set.mem_insert (a b: Object) (X: Set) : a ∈ insert b X ↔ a = b ∨ a ∈ X := by
  sorry

theorem SetTheory.Set.pair_eq (a b:Object) : ({a,b}:Set) = {a} ∪ {b} := by
  sorry

@[simp]
theorem SetTheory.Set.mem_pair (x a b:Object) : x ∈ ({a,b}:Set) ↔ (x = a ∨ x = b) := by
  sorry

@[simp]
theorem SetTheory.Set.mem_triple (x a b c:Object) : x ∈ ({a,b,c}:Set) ↔ (x = a ∨ x = b ∨ x = c) := by
  sorry

theorem SetTheory.Set.singleton_uniq (a:Object) : ∃! (X:Set), ∀ x, x ∈ X ↔ x = a := by
  sorry

theorem SetTheory.Set.pair_uniq (a b:Object) : ∃! (X:Set), ∀ x, x ∈ X ↔ x = a ∨ x = b := by
  sorry

theorem SetTheory.Set.pair_comm (a b:Object) : ({a,b}:Set) = {b,a} := by
  sorry

@[simp]
theorem SetTheory.Set.pair_self (a:Object) : ({a,a}:Set) = {a} := by
  sorry

abbrev SetTheory.Set.empty : Set := ∅
abbrev SetTheory.Set.singleton_empty : Set := {(empty: Object)}
abbrev SetTheory.Set.pair_empty : Set := {(empty: Object), (singleton_empty: Object)}

theorem SetTheory.Set.union_congr_left (A A' B:Set) (h: A = A') : A ∪ B = A' ∪ B := by
  sorry

theorem SetTheory.Set.union_congr_right (A B B':Set) (h: B = B') : A ∪ B = A ∪ B' := by
  sorry

theorem SetTheory.Set.singleton_union_singleton (a b:Object) :
    ({a}:Set) ∪ ({b}:Set) = {a,b} := by
  sorry

theorem SetTheory.Set.union_comm (A B:Set) : A ∪ B = B ∪ A := by
  sorry

theorem SetTheory.Set.union_assoc (A B C:Set) : (A ∪ B) ∪ C = A ∪ (B ∪ C) := by
  sorry

@[simp]
theorem SetTheory.Set.union_self (A:Set) : A ∪ A = A := by
  sorry

@[simp]
theorem SetTheory.Set.union_empty (A:Set) : A ∪ ∅ = A := by
  sorry

@[simp]
theorem SetTheory.Set.empty_union (A:Set) : ∅ ∪ A = A := by
  sorry

theorem SetTheory.Set.triple_eq (a b c:Object) : {a,b,c} = ({a}:Set) ∪ {b,c} := by
  sorry

theorem SetTheory.Set.pair_union_pair (a b c:Object) :
    ({a,b}:Set) ∪ {b,c} = {a,b,c} := by
  sorry

instance SetTheory.Set.instSubset : HasSubset Set where
  Subset X Y := ∀ x, x ∈ X → x ∈ Y

instance SetTheory.Set.instSSubset : HasSSubset Set where
  SSubset X Y := X ⊆ Y ∧ X ≠ Y

theorem SetTheory.Set.subset_def (X Y:Set) : X ⊆ Y ↔ ∀ x, x ∈ X → x ∈ Y := by
  sorry

theorem SetTheory.Set.ssubset_def (X Y:Set) : X ⊂ Y ↔ (X ⊆ Y ∧ X ≠ Y) := by
  sorry

theorem SetTheory.Set.subset_congr_left {A A' B:Set} (hAA':A = A') (hAB: A ⊆ B) : A' ⊆ B := by
  sorry

@[simp, refl]
theorem SetTheory.Set.subset_self (A:Set) : A ⊆ A := by
  sorry

@[simp]
theorem SetTheory.Set.empty_subset (A:Set) : ∅ ⊆ A := by
  sorry

theorem SetTheory.Set.subset_trans {A B C:Set} (hAB:A ⊆ B) (hBC:B ⊆ C) : A ⊆ C := by
  sorry

theorem SetTheory.Set.subset_antisymm (A B:Set) (hAB:A ⊆ B) (hBA:B ⊆ A) : A = B := by
  sorry

theorem SetTheory.Set.ssubset_trans (A B C:Set) (hAB:A ⊂ B) (hBC:B ⊂ C) : A ⊂ C := by
  sorry

abbrev SetTheory.Set.toSubtype (A:Set) := Subtype (fun x ↦ x ∈ A)

instance : CoeSort (Set) (Type v) where
  coe A := A.toSubtype

lemma SetTheory.Set.subtype_property (A:Set) (x:A) : x.val ∈ A := by
  sorry

lemma SetTheory.Set.subtype_coe (A:Set) (x:A) : x.val = x := by
  sorry

lemma SetTheory.Set.coe_inj (A:Set) (x y:A) : x.val = y.val ↔ x = y := by
  sorry

def SetTheory.Set.subtype_mk (A:Set) {x:Object} (hx:x ∈ A) : A := ⟨ x, hx ⟩

@[simp]
lemma SetTheory.Set.subtype_mk_coe {A:Set} {x:Object} (hx:x ∈ A) : A.subtype_mk hx = x := by
  sorry

abbrev SetTheory.Set.specify (A:Set) (P: A → Prop) : Set := SetTheory.specify A P

theorem SetTheory.Set.specification_axiom {A:Set} {P: A → Prop} {x:Object} (h: x ∈ A.specify P) :
    x ∈ A := by
  sorry

theorem SetTheory.Set.specification_axiom' {A:Set} (P: A → Prop) (x:A) :
    x.val ∈ A.specify P ↔ P x := by
  sorry

@[simp]
theorem SetTheory.Set.specification_axiom'' {A:Set} (P: A → Prop) (x:Object) :
    x ∈ A.specify P ↔ ∃ h:x ∈ A, P ⟨ x, h ⟩ := by
  sorry

theorem SetTheory.Set.specify_subset {A:Set} (P: A → Prop) : A.specify P ⊆ A := by
  sorry

instance SetTheory.Set.instIntersection : Inter Set where
  inter X Y := X.specify (fun x ↦ x.val ∈ Y)

@[simp]
theorem SetTheory.Set.mem_inter (x:Object) (X Y:Set) : x ∈ (X ∩ Y) ↔ (x ∈ X ∧ x ∈ Y) := by
  sorry

instance SetTheory.Set.instSDiff : SDiff Set where
  sdiff X Y := X.specify (fun x ↦ x.val ∉ Y)

@[simp]
theorem SetTheory.Set.mem_sdiff (x:Object) (X Y:Set) : x ∈ (X \ Y) ↔ (x ∈ X ∧ x ∉ Y) := by
  sorry

theorem SetTheory.Set.inter_comm (A B:Set) : A ∩ B = B ∩ A := by
  sorry

theorem SetTheory.Set.subset_union {A X: Set} (hAX: A ⊆ X) : A ∪ X = X := by
  sorry

theorem SetTheory.Set.union_subset {A X: Set} (hAX: A ⊆ X) : X ∪ A = X := by
  sorry

@[simp]
theorem SetTheory.Set.inter_self (A:Set) : A ∩ A = A := by
  sorry

theorem SetTheory.Set.inter_assoc (A B C:Set) : (A ∩ B) ∩ C = A ∩ (B ∩ C) := by
  sorry

theorem  SetTheory.Set.inter_union_distrib_left (A B C:Set) :
    A ∩ (B ∪ C) = (A ∩ B) ∪ (A ∩ C) := by
  sorry

theorem  SetTheory.Set.union_inter_distrib_left (A B C:Set) :
    A ∪ (B ∩ C) = (A ∪ B) ∩ (A ∪ C) := by
  sorry

theorem SetTheory.Set.union_compl {A X:Set} (hAX: A ⊆ X) : A ∪ (X \ A) = X := by
  sorry

theorem SetTheory.Set.inter_compl {A X:Set} : A ∩ (X \ A) = ∅ := by
  sorry

theorem SetTheory.Set.compl_union {A B X:Set} : X \ (A ∪ B) = (X \ A) ∩ (X \ B) := by
  sorry

theorem SetTheory.Set.compl_inter {A B X:Set} : X \ (A ∩ B) = (X \ A) ∪ (X \ B) := by
  sorry

instance SetTheory.Set.instDistribLattice : DistribLattice Set where
  le := (· ⊆ ·)
  le_refl := subset_self
  le_trans := fun _ _ _ ↦ subset_trans
  le_antisymm := subset_antisymm
  inf := (· ∩ ·)
  sup := (· ∪ ·)
  le_sup_left := by sorry
  le_sup_right := by sorry
  sup_le := by sorry
  inf_le_left := by sorry
  inf_le_right := by sorry
  le_inf := by sorry
  le_sup_inf := by
    intro X Y Z
    change (X ∪ Y) ∩ (X ∪ Z) ⊆ X ∪ (Y ∩ Z)
    rw [←union_inter_distrib_left]

instance SetTheory.Set.instOrderBot : OrderBot Set where
  bot := ∅
  bot_le := empty_subset

theorem SetTheory.Set.disjoint_iff (A B:Set) : Disjoint A B ↔ A ∩ B = ∅ := by
  sorry

abbrev SetTheory.Set.replace (A:Set) {P: A → Object → Prop}
  (hP : ∀ x y y', P x y ∧ P x y' → y = y') : Set := SetTheory.replace A P hP

@[simp]
theorem SetTheory.Set.replacement_axiom {A:Set} {P: A → Object → Prop}
  (hP: ∀ x y y', P x y ∧ P x y' → y = y') (y:Object) :
    y ∈ A.replace hP ↔ ∃ x, P x y := by
  sorry

abbrev Nat := SetTheory.nat

def SetTheory.Set.nat_equiv : ℕ ≃ Nat := SetTheory.nat_equiv

instance SetTheory.Set.instOfNat {n:ℕ} : OfNat Nat n where
  ofNat := nat_equiv n

instance SetTheory.Set.instNatCast : NatCast Nat where
  natCast n := nat_equiv n

instance SetTheory.Set.toNat : Coe Nat ℕ where
  coe n := nat_equiv.symm n

instance SetTheory.Object.instNatCast : NatCast Object where
  natCast n := (n:Nat).val

instance SetTheory.Object.instOfNat {n:ℕ} : OfNat Object n where
  ofNat := ((n:Nat):Object)

@[simp]
lemma SetTheory.Object.ofnat_eq {n:ℕ} : ((n:Nat):Object) = (n:Object) := by
  sorry

lemma SetTheory.Object.ofnat_eq' {n:ℕ} : (ofNat(n):Object) = (n:Object) := by
  sorry

@[simp]
lemma SetTheory.Object.ofnat_eq'' {n:Nat} : ((n:ℕ):Object) = (n: Object) := by
  sorry

@[simp]
lemma SetTheory.Object.ofnat_eq''' {n:ℕ} {hn} : ((⟨(n:Object), hn⟩: nat): ℕ) = n := by
  sorry

lemma SetTheory.Set.nat_coe_eq {n:ℕ} : (n:Nat) = OfNat.ofNat n := by
  sorry

@[simp]
lemma SetTheory.Set.nat_equiv_inj (n m:ℕ) : (n:Nat) = (m:Nat) ↔ n=m  := by
  sorry

@[simp]
lemma SetTheory.Set.nat_equiv_symm_inj (n m:Nat) : (n:ℕ) = (m:ℕ) ↔ n = m := by
  sorry

@[simp]
theorem SetTheory.Set.ofNat_inj (n m:ℕ) :
    (ofNat(n) : Nat) = (ofNat(m) : Nat) ↔ ofNat(n) = ofNat(m) := by
  sorry

@[simp]
theorem SetTheory.Set.ofNat_inj' (n m:ℕ) :
    (ofNat(n) : Object) = (ofNat(m) : Object) ↔ ofNat(n) = ofNat(m) := by
  sorry

@[simp]
lemma SetTheory.Set.nat_coe_eq_iff {m n : ℕ} : (m:Object) = ofNat(n) ↔ m = n := by
  sorry

@[simp]
theorem SetTheory.Object.natCast_inj (n m:ℕ) :
    (n : Object) = (m : Object) ↔ n = m := by
  sorry

@[simp]
lemma SetTheory.Set.nat_equiv_coe_of_coe (n:ℕ) : ((n:Nat):ℕ) = n := by
  sorry

@[simp]
lemma SetTheory.Set.nat_equiv_coe_of_coe' (n:Nat) : ((n:ℕ):Nat) = n := by
  sorry

@[simp]
lemma SetTheory.Set.nat_equiv_coe_of_coe'' (n:ℕ) : ((ofNat(n):Nat):ℕ) = n := by
  sorry

@[simp]
lemma SetTheory.Set.nat_coe_eq_iff' {m: Nat} {n : ℕ} : (m:Object) = (ofNat(n):Object) ↔ (m:ℕ) = ofNat(n) := by
  sorry

instance SetTheory.Set.inst_coe_set : Coe Set (_root_.Set Object) where
  coe X := { x | x ∈ X }

@[simp]
theorem SetTheory.Set.coe_inj' (X Y:Set) :
    (X : _root_.Set Object) = (Y : _root_.Set Object) ↔ X = Y := by
  sorry

theorem SetTheory.Set.mem_coe (X:Set) (x:Object) : x ∈ (X : _root_.Set Object) ↔ x ∈ X := by
  sorry

theorem SetTheory.Set.coe_empty : ((∅:Set) : _root_.Set Object) = ∅ := by
  sorry

theorem SetTheory.Set.coe_subset (X Y:Set) :
    (X : _root_.Set Object) ⊆ (Y : _root_.Set Object) ↔ X ⊆ Y := by
  sorry

theorem SetTheory.Set.coe_ssubset (X Y:Set) :
    (X : _root_.Set Object) ⊂ (Y : _root_.Set Object) ↔ X ⊂ Y := by
  sorry

theorem SetTheory.Set.coe_singleton (x: Object) : (({x}:Set) : _root_.Set Object) = {x} := by
  sorry

theorem SetTheory.Set.coe_union (X Y: Set) :
    ((X ∪ Y:Set) : _root_.Set Object) = (X : _root_.Set Object) ∪ (Y : _root_.Set Object) := by
  sorry

theorem SetTheory.Set.coe_pair (x y: Object) : (({x, y}:Set) : _root_.Set Object) = {x, y} := by
  sorry

theorem SetTheory.Set.coe_subtype (X: Set) :  (X : _root_.Set Object) = X.toSubtype := by
  sorry

theorem SetTheory.Set.coe_intersection (X Y: Set) :
    ((X ∩ Y:Set) : _root_.Set Object) = (X : _root_.Set Object) ∩ (Y : _root_.Set Object) := by
  sorry

theorem SetTheory.Set.coe_diff (X Y: Set) :
    ((X \ Y:Set) : _root_.Set Object) = (X : _root_.Set Object) \ (Y : _root_.Set Object) := by
  sorry

theorem SetTheory.Set.coe_Disjoint (X Y: Set) :
    Disjoint (X : _root_.Set Object) (Y : _root_.Set Object) ↔ Disjoint X Y := by
  sorry

abbrev SetTheory.Set.image {X Y:Set} (f:X → Y) (S: Set) : Set :=
  X.replace (P := fun x y ↦ f x = y ∧ x.val ∈ S) (by simp_all)

theorem SetTheory.Set.mem_image {X Y:Set} (f:X → Y) (S: Set) (y:Object) :
    y ∈ image f S ↔ ∃ x:X, x.val ∈ S ∧ f x = y := by
  sorry

theorem SetTheory.Set.image_eq_specify {X Y:Set} (f:X → Y) (S: Set) :
    image f S = Y.specify (fun y ↦ ∃ x:X, x.val ∈ S ∧ f x = y) := by
  sorry

theorem SetTheory.Set.image_eq_image {X Y:Set} (f:X → Y) (S: Set):
    (image f S: _root_.Set Object) = Subtype.val '' (f '' {x | x.val ∈ S}) := by
  sorry

theorem SetTheory.Set.image_in_codomain {X Y:Set} (f:X → Y) (S: Set) :
    image f S ⊆ Y := by
  sorry

abbrev f_3_4_2 : nat → nat := fun n ↦ (2*n:ℕ)

theorem SetTheory.Set.image_f_3_4_2 : image f_3_4_2 {1,2,3} = {2,4,6} := by
  sorry

theorem SetTheory.Set.mem_image_of_eval {X Y:Set} (f:X → Y) (S: Set) (x:X) :
    x.val ∈ S → (f x).val ∈ image f S := by
  sorry

theorem SetTheory.Set.mem_image_of_eval_counter :
    ∃ (X Y:Set) (f:X → Y) (S: Set) (x:X), ¬((f x).val ∈ image f S → x.val ∈ S) := by
  sorry

abbrev SetTheory.Set.preimage {X Y:Set} (f:X → Y) (U: Set) : Set := X.specify (P := fun x ↦ (f x).val ∈ U)

@[simp]
theorem SetTheory.Set.mem_preimage {X Y:Set} (f:X → Y) (U: Set) (x:X) :
    x.val ∈ preimage f U ↔ (f x).val ∈ U := by
  sorry

theorem SetTheory.Set.mem_preimage' {X Y:Set} (f:X → Y) (U: Set) (x:Object) :
    x ∈ preimage f U ↔ ∃ x': X, x'.val = x ∧ (f x').val ∈ U := by
  sorry

theorem SetTheory.Set.preimage_eq {X Y:Set} (f:X → Y) (U: Set) :
    ((preimage f U): _root_.Set Object) = Subtype.val '' (f⁻¹' {y | y.val ∈ U}) := by
  sorry

theorem SetTheory.Set.preimage_in_domain {X Y:Set} (f:X → Y) (U: Set) :
    (preimage f U) ⊆ X := by
  sorry

theorem SetTheory.Set.preimage_f_3_4_2 : preimage f_3_4_2 {2,4,6} = {1,2,3} := by
  sorry

theorem SetTheory.Set.image_preimage_f_3_4_2 :
    image f_3_4_2 (preimage f_3_4_2 {1,2,3}) ≠ {1,2,3} := by
  sorry

instance SetTheory.Set.inst_pow : Pow Set Set where
  pow := pow

@[coe]
def SetTheory.Set.coe_of_fun {X Y:Set} (f: X → Y) : Object := function_to_object X Y f

instance SetTheory.Set.inst_coe_of_fun {X Y:Set} : CoeOut (X → Y) Object where
  coe := coe_of_fun

@[simp]
theorem SetTheory.Set.coe_of_fun_inj {X Y:Set} (f g:X → Y) : (f:Object) = (g:Object) ↔ f = g := by
  sorry

@[simp]
theorem SetTheory.Set.powerset_axiom {X Y:Set} (F:Object) :
    F ∈ (X ^ Y) ↔ ∃ f: Y → X, f = F := by
  sorry

abbrev f_3_4_9_a : ({4,7}:Set) → ({0,1}:Set) := fun x ↦ ⟨ 0, by simp ⟩

open Classical in
noncomputable abbrev f_3_4_9_b : ({4,7}:Set) → ({0,1}:Set) :=
  fun x ↦ if x.val = 4 then ⟨ 0, by simp ⟩ else ⟨ 1, by simp ⟩

open Classical in
noncomputable abbrev f_3_4_9_c : ({4,7}:Set) → ({0,1}:Set) :=
  fun x ↦ if x.val = 4 then ⟨ 1, by simp ⟩ else ⟨ 0, by simp ⟩

abbrev f_3_4_9_d : ({4,7}:Set) → ({0,1}:Set) := fun x ↦ ⟨ 1, by simp ⟩

theorem SetTheory.Set.example_3_4_9 (F:Object) :
    F ∈ ({0,1}:Set) ^ ({4,7}:Set) ↔ F = f_3_4_9_a
    ∨ F = f_3_4_9_b ∨ F = f_3_4_9_c ∨ F = f_3_4_9_d := by
  sorry

def SetTheory.Set.powerset (X:Set) : Set :=
  (({0,1} ^ X): Set).replace (P := sorry) (by sorry)

theorem SetTheory.Set.exists_powerset (X:Set) :
   ∃ (Z: Set), ∀ x, x ∈ Z ↔ ∃ Y:Set, x = Y ∧ Y ⊆ X := by
  sorry

theorem SetTheory.Set.powerset_of_triple (a b c x:Object) :
    x ∈ powerset {a,b,c}
    ↔ x = (∅:Set)
    ∨ x = ({a}:Set)
    ∨ x = ({b}:Set)
    ∨ x = ({c}:Set)
    ∨ x = ({a,b}:Set)
    ∨ x = ({a,c}:Set)
    ∨ x = ({b,c}:Set)
    ∨ x = ({a,b,c}:Set) := by
  sorry

theorem SetTheory.Set.union_axiom (A: Set) (x:Object) :
    x ∈ union A ↔ ∃ (S:Set), x ∈ S ∧ (S:Object) ∈ A := by
  sorry

theorem SetTheory.Set.example_3_4_12 :
    union { (({2,3}:Set):Object), (({3,4}:Set):Object), (({4,5}:Set):Object) } = {2,3,4,5} := by
  sorry

theorem SetTheory.Set.union_eq (A: Set) :
    (union A : _root_.Set Object) =
    ⋃₀ { S : _root_.Set Object | ∃ S':Set, S = S' ∧ (S':Object) ∈ A } := by
  sorry

abbrev SetTheory.Set.iUnion (I: Set) (A: I → Set) : Set :=
  union (I.replace (P := fun α S ↦ S = A α) (by intro _ _ _ ⟨h1, h2⟩; exact h1.trans h2.symm))

theorem SetTheory.Set.mem_iUnion {I:Set} (A: I → Set) (x:Object) :
    x ∈ iUnion I A ↔ ∃ α:I, x ∈ A α := by
  sorry

open Classical in
noncomputable abbrev SetTheory.Set.index_example : ({1,2,3}:Set) → Set :=
  fun i ↦ if i.val = 1 then {2,3} else if i.val = 2 then {3,4} else {4,5}

theorem SetTheory.Set.iUnion_example : iUnion {1,2,3} index_example = {2,3,4,5} := by
  sorry

theorem SetTheory.Set.iUnion_eq (I: Set) (A: I → Set) :
    (iUnion I A : _root_.Set Object) = ⋃ α, (A α: _root_.Set Object) := by
  sorry

theorem SetTheory.Set.iUnion_of_empty (A: (∅:Set) → Set) : iUnion (∅:Set) A = ∅ := by
  sorry

noncomputable abbrev SetTheory.Set.nonempty_choose {I:Set} (hI: I ≠ ∅) : I :=
  ⟨(nonempty_def hI).choose, (nonempty_def hI).choose_spec⟩

abbrev SetTheory.Set.iInter' (I:Set) (β:I) (A: I → Set) : Set :=
  (A β).specify (P := fun x ↦ ∀ α:I, x.val ∈ A α)

noncomputable abbrev SetTheory.Set.iInter (I: Set) (hI: I ≠ ∅) (A: I → Set) : Set :=
  iInter' I (nonempty_choose hI) A

theorem SetTheory.Set.mem_iInter {I:Set} (hI: I ≠ ∅) (A: I → Set) (x:Object) :
    x ∈ iInter I hI A ↔ ∀ α:I, x ∈ A α := by
  sorry

def SetTheory.Set.image_of_inter' : Decidable (∀ X Y:Set, ∀ f:X → Y, ∀ A B: Set, image f (A ∩ B) = (image f A) ∩ (image f B)) := by
  sorry

def SetTheory.Set.image_of_diff' : Decidable (∀ X Y:Set, ∀ f:X → Y, ∀ A B: Set, image f (A \ B) = (image f A) \ (image f B)) := by
  sorry

theorem SetTheory.Set.union_of_nonempty {I J:Set} (hI: I ≠ ∅) (hJ: J ≠ ∅) : I ∪ J ≠ ∅ := by
  sorry
-- </CONTEXT>
-- <TARGET_THEOREM>
theorem SetTheory.Set.inter_iInter {I J:Set} (hI: I ≠ ∅) (hJ: J ≠ ∅) (A: (I ∪ J:Set) → Set) :
    iInter I hI (fun α ↦ A ⟨ α.val, by simp [α.property]⟩)
    ∩ iInter J hJ (fun α ↦ A ⟨ α.val, by simp [α.property]⟩)
    = iInter (I ∪ J) (union_of_nonempty hI hJ) A := by
-- </TARGET_THEOREM>

end Chapter3