import Mathlib

namespace Chapter8

-- <CONTEXT>
abbrev EqualCard (X Y : Type) : Prop := ∃ f : X → Y, Function.Bijective f

theorem EqualCard.iff {X Y : Type} : EqualCard X Y ↔ Nonempty (X ≃ Y) := by
  sorry

theorem EqualCard.iff' {X Y : Type} : EqualCard X Y ↔ Cardinal.mk X = Cardinal.mk Y := by
  sorry

theorem EqualCard.refl (X : Type) : EqualCard X X := by
  sorry

theorem EqualCard.symm {X Y : Type} (hXY : EqualCard X Y) : EqualCard Y X := by
  sorry

theorem EqualCard.trans {X Y Z : Type} (hXY : EqualCard X Y) (hYZ : EqualCard Y Z) :
  EqualCard X Z := by
  sorry

instance EqualCard.instSetoid : Setoid Type := ⟨ EqualCard, ⟨ refl, symm, trans ⟩ ⟩

theorem EqualCard.univ (X : Type) : EqualCard (.univ : Set X) X := by
  sorry

abbrev CountablyInfinite (X : Type) : Prop := EqualCard X ℕ

abbrev AtMostCountable (X : Type) : Prop := CountablyInfinite X ∨ Finite X

theorem CountablyInfinite.equiv {X Y: Type} (hXY : EqualCard X Y) :
  CountablyInfinite X ↔ CountablyInfinite Y := by
  sorry

theorem Finite.equiv {X Y: Type} (hXY : EqualCard X Y) :
  Finite X ↔ Finite Y := by
  sorry

theorem AtMostCountable.equiv {X Y: Type} (hXY : EqualCard X Y) :
  AtMostCountable X ↔ AtMostCountable Y := by
  sorry

theorem CountablyInfinite.iff (X : Type) : CountablyInfinite X ↔ Nonempty (Denumerable X) := by
  sorry

theorem CountablyInfinite.iff' (X : Type) : CountablyInfinite X ↔ Countable X ∧ Infinite X := by
  sorry

theorem CountablyInfinite.toCountable {X : Type} (hX: CountablyInfinite X) : Countable X := by
  sorry

theorem CountablyInfinite.toInfinite {X : Type} (hX: CountablyInfinite X) : Infinite X := by
  sorry

theorem AtMostCountable.iff (X : Type) : AtMostCountable X ↔ Countable X := by
  sorry

theorem CountablyInfinite.iff_image_inj {A:Type} (X: Set A) : CountablyInfinite X ↔ ∃ f : ℕ ↪ A, X = f '' .univ := by
  sorry

theorem Nat.exists_unique_min {X : Set ℕ} (hX : X.Nonempty) :
  ∃! m ∈ X, ∀ n ∈ X, m ≤ n := by
  sorry

def Int.exists_unique_min : Decidable (∀ (X : Set ℤ) (hX : X.Nonempty), ∃! m ∈ X, ∀ n ∈ X, m ≤ n) := by
  sorry

def NNRat.exists_unique_min : Decidable (∀ (X : Set NNRat) (hX : X.Nonempty), ∃! m ∈ X, ∀ n ∈ X, m ≤ n) := by
  sorry

open Classical in
noncomputable abbrev Nat.min (X : Set ℕ) : ℕ := if hX : X.Nonempty then (exists_unique_min hX).exists.choose else 0

theorem Nat.min_spec {X : Set ℕ} (hX : X.Nonempty) : min X ∈ X ∧ ∀ n ∈ X, min X ≤ n := by
  sorry

theorem Nat.min_eq {X : Set ℕ} (hX : X.Nonempty) {a:ℕ} (ha : a ∈ X ∧ ∀ n ∈ X, a ≤ n) : min X = a := by
  sorry

@[simp]
theorem Nat.min_empty : min ∅ = 0 := by
  sorry

theorem Nat.min_eq_sInf {X : Set ℕ} (hX : X.Nonempty) : min X = sInf X := by
  sorry

open Classical in
theorem Nat.min_eq_find {X : Set ℕ} (hX : X.Nonempty) : min X = Nat.find hX := by
  sorry

theorem Nat.monotone_enum_of_infinite (X : Set ℕ) [Infinite X] : ∃! f : ℕ → X, Function.Bijective f ∧ StrictMono f := by
  sorry

theorem Nat.countable_of_infinite (X : Set ℕ) [Infinite X] : CountablyInfinite X := by
  sorry

theorem Nat.atMostCountable_subset (X: Set ℕ) : AtMostCountable X := by
  sorry

theorem AtMostCountable.subset {X: Type} (hX : AtMostCountable X) (Y: Set X) : AtMostCountable Y := by
  sorry

theorem AtMostCountable.subset' {A: Type} {X Y: Set A} (hX: AtMostCountable X) (hY: Y ⊆ X) : AtMostCountable Y := by
  sorry

theorem Int.countablyInfinite : CountablyInfinite ℤ := by
  sorry

theorem CountablyInfinite.lower_diag : CountablyInfinite { n : ℕ × ℕ | n.2 ≤ n.1 } := by
  sorry

theorem CountablyInfinite.prod_nat : CountablyInfinite (ℕ × ℕ) := by
  sorry

theorem Rat.countablyInfinite : CountablyInfinite ℚ := by
  sorry

abbrev explicit_bijection : ℕ → ℚ := sorry

theorem explicit_bijection_spec : Function.Bijective explicit_bijection := by
  sorry
-- </CONTEXT>
-- <TARGET_THEOREM>
theorem Exercise_8_1_9 {I X:Type} (hI: AtMostCountable I) (A: I → Set X) (hA: ∀ i, AtMostCountable (A i)) :
  AtMostCountable (⋃ i, A i) := by
-- </TARGET_THEOREM>

end Chapter8