import Mathlib.Tactic

/-!
Self-contained slice for Chapter 3 Section 3.3 target theorem.
-/

namespace Chapter3

universe u v

/-- The axioms of Zermelo-Frankel theory with atoms.  -/
class SetTheory where
  Set : Type u -- Axiom 3.1
  Object : Type v -- Axiom 3.1
  set_to_object : Set ↪ Object -- Axiom 3.1
  mem : Object → Set → Prop -- Axiom 3.1
  extensionality X Y : (∀ x, mem x X ↔ mem x Y) → X = Y -- Axiom 3.2
  emptyset: Set -- Axiom 3.3
  emptyset_mem x : ¬ mem x emptyset -- Axiom 3.3
  singleton : Object → Set -- Axiom 3.4
  singleton_axiom x y : mem x (singleton y) ↔ x = y -- Axiom 3.4
  union_pair : Set → Set → Set -- Axiom 3.5
  union_pair_axiom X Y x : mem x (union_pair X Y) ↔ (mem x X ∨ mem x Y) -- Axiom 3.5
  specify A (P: Subtype (mem . A) → Prop) : Set -- Axiom 3.6
  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 -- Axiom 3.6
  replace A (P: Subtype (mem . A) → Object → Prop)
    (hP: ∀ x y y', P x y ∧ P x y' → y = y') : Set -- Axiom 3.7
  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 -- Axiom 3.7
  nat : Set -- Axiom 3.8
  nat_equiv : ℕ ≃ Subtype (mem . nat) -- Axiom 3.8
  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 -- Axiom 3.9
  pow : Set → Set → Set -- Axiom 3.11
  function_to_object (X: Set) (Y: Set) :
    (Subtype (mem . X) → Subtype (mem . Y)) ↪ Object -- Axiom 3.11
  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 -- Axiom 3.11
  union : Set → Set -- Axiom 3.12
  union_axiom A x : mem x (union A) ↔ ∃ S, mem x S ∧ mem (set_to_object S) A -- Axiom 3.12

-- This enables one to use `Set` and `Object` instead of `SetTheory.Set` and `SetTheory.Object`.
export SetTheory (Set Object)

-- This instance implicitly imposes the axioms of Zermelo-Frankel set theory with atoms.
variable [SetTheory]

/-- Definition 3.1.1 (objects can be elements of sets) -/
instance SetTheory.objects_mem_sets : Membership Object Set where
  mem X x := SetTheory.mem x X

/--
  This defines the subtype `A.toSubtype` for any `A:Set`.
  Note that `A.toSubtype` gives you a type, similar to how `Object` or `Set` are types.
  A value `x'` of type `A.toSubtype` combines some `x: Object` with a proof that `hx: x ∈ A`.

  To produce an element `x'` of this subtype, use `⟨ x, hx ⟩`, where `x: Object` and `hx: x ∈ A`.
  The object `x` associated to a subtype element `x'` is recovered as `x'.val`, and
  the property `hx` that `x` belongs to `A` is recovered as `x'.property`.
-/
abbrev SetTheory.Set.toSubtype (A:Set) := Subtype (fun x ↦ x ∈ A)

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

end Chapter3

/-! Tools.ExistsUnique (copied) -/

noncomputable def ExistsUnique.choose {α: Sort*} {p: α → Prop} (h : ∃! x, p x) : α := h.exists.choose

theorem ExistsUnique.choose_spec {α: Sort*} {p: α → Prop} (h : ∃! x, p x) :
  p h.choose := h.exists.choose_spec

theorem ExistsUnique.choose_eq {α: Sort*} {p: α → Prop} (h : ∃! x, p x) {x : α} (hx : p x) :
  h.choose = x := h.unique h.choose_spec hx

/-- Alternate characterization useful in Section_3_3 -/
theorem ExistsUnique.choose_iff {α: Sort*} {p: α → Prop} (h : ∃! x, p x) (x : α):
  p x ↔ x = h.choose :=
  ⟨ by intro hx; exact (h.choose_eq hx).symm, by rintro rfl; exact h.choose_spec ⟩

namespace Chapter3

export SetTheory (Set Object)
variable [SetTheory]

/--
  Definition 3.3.1. `Function X Y` is the structure of functions from `X` to `Y`.
  Analogous to the Mathlib type `X → Y`.
-/
@[ext]
structure Function (X Y: Set) where
  P : X → Y → Prop
  unique : ∀ x: X, ∃! y: Y, P x y

/--
  Converting a Chapter 3 function `f: Function X Y` to a Mathlib function `f: X → Y`.
  The Chapter 3 definition of a function was nonconstructive, so we have to use the
  axiom of choice here.
-/
noncomputable def Function.to_fn {X Y: Set} (f: Function X Y) : X → Y :=
  fun x ↦ (f.unique x).choose

noncomputable instance Function.inst_coefn (X Y: Set) : CoeFun (Function X Y) (fun _ ↦ X → Y) where
  coe := Function.to_fn

/-- Converting a Mathlib function to a Chapter 3 `Function` -/
abbrev Function.mk_fn {X Y: Set} (f: X → Y) : Function X Y :=
  Function.mk (fun x y ↦ y = f x) (by simp)

/-- Definition 3.3.13 (Composition) -/
noncomputable abbrev Function.comp {X Y Z: Set} (g: Function Y Z) (f: Function X Y) :
    Function X Z :=
  Function.mk_fn (fun x ↦ g (f x))

-- `∘` is already taken in Mathlib for the composition of Mathlib functions,
-- so we use `○` here instead to avoid ambiguity.
infix:90 "○" => Function.comp

abbrev Function.one_to_one {X Y: Set} (f: Function X Y) : Prop := ∀ x x': X, x ≠ x' → f x ≠ f x'

/--
  Exercise 3.3.4.
-/
theorem Function.comp_cancel_left {X Y Z:Set} {f f': Function X Y} {g : Function Y Z}
  (heq : g ○ f = g ○ f') (hg: g.one_to_one) : f = f' := by
end Chapter3