{-# OPTIONS --safe --warning=error --without-K #-}

open import Functions
open import Agda.Primitive using (Level; lzero; lsuc; _⊔_)
open import LogicalFormulae
open import Numbers.Naturals.Naturals
open import Numbers.Naturals.Definition
open import Numbers.Naturals.Order
open import Orders

module Sets.FinSet where
  data FinSet : (n : ℕ) → Set where
    fzero : {n : ℕ} → FinSet (succ n)
    fsucc : {n : ℕ} → FinSet n → FinSet (succ n)

  data FinNotEquals : {n : ℕ} → (a b : FinSet (succ n)) → Set where
    fne2 : (a b : FinSet 2) → ((a ≡ fzero) && (b ≡ fsucc (fzero))) || ((b ≡ fzero) && (a ≡ fsucc (fzero))) → FinNotEquals {1} a b
    fneN : {n : ℕ} → (a b : FinSet (succ (succ (succ n)))) → (((a ≡ fzero) && (Sg (FinSet (succ (succ n))) (λ c → b ≡ fsucc c))) || ((Sg (FinSet (succ (succ n))) (λ c → a ≡ fsucc c)) && (b ≡ fzero))) || (Sg (FinSet (succ (succ n)) && FinSet (succ (succ n))) (λ t → (a ≡ fsucc (_&&_.fst t)) & (b ≡ fsucc (_&&_.snd t)) & FinNotEquals (_&&_.fst t) (_&&_.snd t))) → FinNotEquals {succ (succ n)} a b

  fsuccInjective : {n : ℕ} → {a b : FinSet n} → fsucc a ≡ fsucc b → a ≡ b
  fsuccInjective refl = refl

  fneSymmetric : {n : ℕ} → {a b : FinSet (succ n)} → FinNotEquals a b → FinNotEquals b a
  fneSymmetric {.1} {.a1} {.b1} (fne2 a1 b1 (inl x)) = fne2 b1 a1 (inr x)
  fneSymmetric {.1} {.a1} {.b1} (fne2 a1 b1 (inr x)) = fne2 b1 a1 (inl x)
  fneSymmetric {.(succ (succ _))} {.fzero} {b} (fneN .fzero b (inl (inl (refl ,, snd)))) = fneN b fzero (inl (inr (snd ,, refl)))
  fneSymmetric {.(succ (succ _))} {a} {.fzero} (fneN a .fzero (inl (inr (fst ,, refl)))) = fneN fzero a (inl (inl (refl ,, fst)))
  fneSymmetric {.(succ (succ _))} {a} {b} (fneN a b (inr ((fst ,, snd) , record { one = o ; two = p ; three = q }))) = fneN b a (inr ((snd ,, fst) , record { one = p ; two = o ; three = fneSymmetric q }))

  underlyingProof : {l m : _} {L : Set l} {pr : L → Set m} → (a : Sg L pr) → pr (underlying a)
  underlyingProof (a , b) = b

  sgEq : {l m : _} {L : Set l} → {pr : L → Set m} → {a b : Sg L pr} → (underlying a ≡ underlying b) → ({c : L} → (r s : pr c) → r ≡ s) → (a ≡ b)
  sgEq {l} {m} {L} {prop} {(a , b1)} {(.a , b)} refl pr2 with pr2 {a} b b1
  sgEq {l} {m} {L} {prop} {(a , b1)} {(.a , .b1)} refl pr2 | refl = refl


  finSetNotEquals : {n : ℕ} → {a b : FinSet (succ n)} → (a ≡ b → False) → FinNotEquals {n} a b
  finSetNotEquals {zero} {fzero} {fzero} pr = exFalso (pr refl)
  finSetNotEquals {zero} {fzero} {fsucc ()} pr
  finSetNotEquals {zero} {fsucc ()} {b} pr
  finSetNotEquals {succ zero} {fzero} {fzero} pr = exFalso (pr refl)
  finSetNotEquals {succ zero} {fzero} {fsucc fzero} pr = fne2 fzero (fsucc fzero) (inl (refl ,, refl))
  finSetNotEquals {succ zero} {fzero} {fsucc (fsucc ())} pr
  finSetNotEquals {succ zero} {fsucc fzero} {fzero} pr = fne2 (fsucc fzero) fzero (inr (refl ,, refl))
  finSetNotEquals {succ zero} {fsucc fzero} {fsucc fzero} pr = exFalso (pr refl)
  finSetNotEquals {succ zero} {fsucc fzero} {fsucc (fsucc ())} pr
  finSetNotEquals {succ zero} {fsucc (fsucc ())}
  finSetNotEquals {succ (succ n)} {fzero} {fzero} pr = exFalso (pr refl)
  finSetNotEquals {succ (succ n)} {fzero} {fsucc b} pr = fneN fzero (fsucc b) (inl (inl (refl ,, (b , refl))))
  finSetNotEquals {succ (succ n)} {fsucc a} {fzero} pr = fneN (fsucc a) fzero (inl (inr ((a , refl) ,, refl)))
  finSetNotEquals {succ (succ n)} {fsucc a} {fsucc b} pr = fneN (fsucc a) (fsucc b) (inr ans)
    where
      q : a ≡ b → False
      q refl = pr refl
      t : FinNotEquals {succ n} a b
      t = finSetNotEquals {succ n} {a} {b} q
      ans : Sg (FinSet (succ (succ n)) && FinSet (succ (succ n))) (λ x → (fsucc a ≡ fsucc (_&&_.fst x)) & fsucc b ≡ fsucc (_&&_.snd x) & FinNotEquals (_&&_.fst x) (_&&_.snd x))
      ans with t
      ans | fne2 .fzero .(fsucc fzero) (inl (refl ,, refl)) = (a ,, b) , record { one = refl ; two = refl ; three = fne2 fzero (fsucc fzero) (inl (refl ,, refl)) }
      ans | fne2 .(fsucc fzero) .fzero (inr (refl ,, refl)) = (a ,, b) , record { one = refl ; two = refl ; three = fne2 (fsucc fzero) fzero (inr (refl ,, refl)) }
      ans | fneN a b _ = (a ,, b) , record { one = refl ; two = refl ; three = t }

  fzeroNotFsucc : {n : ℕ} → {a : _} → fzero ≡ fsucc {succ n} a → False
  fzeroNotFsucc ()

  finSetNotEqualsSame : {n : ℕ} → {a : FinSet (succ n)} → FinNotEquals a a → False
  finSetNotEqualsSame {.1} {fzero} (fne2 .fzero .fzero (inl (fst ,, ())))
  finSetNotEqualsSame {.1} {fzero} (fne2 .fzero .fzero (inr (fst ,, ())))
  finSetNotEqualsSame {.(succ (succ _))} {fzero} (fneN .fzero .fzero (inl (inl (refl ,, (a , ())))))
  finSetNotEqualsSame {.(succ (succ _))} {fzero} (fneN .fzero .fzero (inl (inr ((a , ()) ,, refl))))
  finSetNotEqualsSame {.(succ (succ _))} {fzero} (fneN .fzero .fzero (inr ((fst ,, snd) , record { one = () ; two = p ; three = q })))
  finSetNotEqualsSame {.1} {fsucc a} (fne2 .(fsucc a) .(fsucc a) (inl (() ,, snd)))
  finSetNotEqualsSame {.1} {fsucc a} (fne2 .(fsucc a) .(fsucc a) (inr (() ,, snd)))
  finSetNotEqualsSame {.(succ (succ _))} {fsucc a} (fneN .(fsucc a) .(fsucc a) (inl (inl (() ,, snd))))
  finSetNotEqualsSame {.(succ (succ _))} {fsucc a} (fneN .(fsucc a) .(fsucc a) (inl (inr (fst ,, ()))))
  finSetNotEqualsSame {.(succ (succ _))} {fsucc a} (fneN .(fsucc a) .(fsucc a) (inr ((.a ,, .a) , record { one = refl ; two = refl ; three = q }))) = finSetNotEqualsSame q

  finNotEqualsFsucc : {n : ℕ} → {a b : FinSet (succ n)} → FinNotEquals (fsucc a) (fsucc b) → FinNotEquals a b
  finNotEqualsFsucc {.0} {a} {b} (fne2 .(fsucc a) .(fsucc b) (inl (() ,, snd)))
  finNotEqualsFsucc {.0} {a} {b} (fne2 .(fsucc a) .(fsucc b) (inr (() ,, snd)))
  finNotEqualsFsucc {n} {a} {b} (fneN .(fsucc a) .(fsucc b) (inl (inl (() ,, snd))))
  finNotEqualsFsucc {n} {a} {b} (fneN .(fsucc a) .(fsucc b) (inl (inr (fst ,, ()))))
  finNotEqualsFsucc {n} {a} {b} (fneN .(fsucc a) .(fsucc b) (inr ((fst ,, snd) , prf))) = identityOfIndiscernablesRight FinNotEquals ans (equalityCommutative b=snd)
    where
      a=fst : a ≡ fst
      a=fst = fsuccInjective (_&_&_.one prf)
      b=snd : b ≡ snd
      b=snd = fsuccInjective (_&_&_.two prf)
      ans : FinNotEquals a snd
      ans = identityOfIndiscernablesLeft FinNotEquals (_&_&_.three prf) (equalityCommutative a=fst)

  finSetNotEquals' : {n : ℕ} → {a b : FinSet (succ n)} → FinNotEquals a b → (a ≡ b → False)
  finSetNotEquals' {n} {a} {.a} fne refl = finSetNotEqualsSame fne

  finset0Empty : FinSet zero → False
  finset0Empty ()

  finset1OnlyOne : (a b : FinSet 1) → a ≡ b
  finset1OnlyOne fzero fzero = refl
  finset1OnlyOne fzero (fsucc ())
  finset1OnlyOne (fsucc ()) b

  intoSmaller : {n m : ℕ} → (n <N m) → (FinSet n → FinSet m)
  intoSmaller {zero} {m} n<m = t
    where
      t : FinSet 0 → FinSet m
      t ()
  intoSmaller {succ n} {zero} ()
  intoSmaller {succ n} {succ m} n<m with intoSmaller (canRemoveSuccFrom<N n<m)
  intoSmaller {succ n} {succ m} n<m | prev = t
    where
      t : FinSet (succ n) → FinSet (succ m)
      t fzero = fzero
      t (fsucc arg) = fsucc (prev arg)

  intoSmallerInj : {n m : ℕ} → (n<m : n <N m) → Injection (intoSmaller n<m)
  intoSmallerInj {zero} {zero} (le x ())
  intoSmallerInj {zero} {succ m} n<m = record { property = inj }
    where
      t : FinSet 0 → FinSet (succ m)
      t ()
      inj : {x y : FinSet zero} → intoSmaller (succIsPositive m) x ≡ intoSmaller (succIsPositive m) y → x ≡ y
      inj {()} {y}
  intoSmallerInj {succ n} {zero} ()
  intoSmallerInj {succ n} {succ m} n<m with intoSmallerInj (canRemoveSuccFrom<N n<m)
  intoSmallerInj {succ n} {succ m} n<m | prevInj = inj
    where
      inj : Injection (intoSmaller n<m)
      Injection.property inj {fzero} {fzero} pr = refl
      Injection.property inj {fzero} {fsucc y} ()
      Injection.property inj {fsucc x} {fzero} ()
      Injection.property inj {fsucc x} {fsucc y} pr = applyEquality fsucc (Injection.property prevInj (fsuccInjective pr))

  toNat : {n : ℕ} → (a : FinSet n) → ℕ
  toNat {.(succ _)} fzero = 0
  toNat {.(succ _)} (fsucc a) = succ (toNat a)

  toNatSmaller : {n : ℕ} → (a : FinSet n) → toNat a <N n
  toNatSmaller {zero} ()
  toNatSmaller {succ n} fzero = succIsPositive n
  toNatSmaller {succ n} (fsucc a) = succPreservesInequality (toNatSmaller a)

  ofNat : {n : ℕ} → (m : ℕ) → (m <N n) → FinSet n
  ofNat {zero} zero (le x ())
  ofNat {succ n} zero m<n = fzero
  ofNat {zero} (succ m) (le x ())
  ofNat {succ n} (succ m) m<n = fsucc (ofNat {n} m (canRemoveSuccFrom<N m<n))

  ofNatInjective : {n : ℕ} → (x y : ℕ) → (pr : x <N n) → (pr2 : y <N n) → ofNat x pr ≡ ofNat y pr2 → x ≡ y
  ofNatInjective {zero} zero zero () y<n pr
  ofNatInjective {zero} zero (succ y) () y<n pr
  ofNatInjective {zero} (succ x) zero x<n () pr
  ofNatInjective {zero} (succ x) (succ y) () y<n pr
  ofNatInjective {succ n} zero zero x<n y<n pr = refl
  ofNatInjective {succ n} zero (succ y) x<n y<n ()
  ofNatInjective {succ n} (succ x) zero x<n y<n ()
  ofNatInjective {succ n} (succ x) (succ y) x<n y<n pr = applyEquality succ (ofNatInjective x y (canRemoveSuccFrom<N x<n) (canRemoveSuccFrom<N y<n) (fsuccInjective pr))

  toNatOfNat : {n : ℕ} → (a : ℕ) → (a<n : a <N n) → toNat (ofNat a a<n) ≡ a
  toNatOfNat {zero} zero (le x ())
  toNatOfNat {zero} (succ a) (le x ())
  toNatOfNat {succ n} zero a<n = refl
  toNatOfNat {succ n} (succ a) a<n = applyEquality succ (toNatOfNat a (canRemoveSuccFrom<N a<n))

  intoSmallerLemm : {n m : ℕ} → {n<m : n <N (succ m)} → {m<sm : m <N succ m} → (b : FinSet n) → intoSmaller n<m b ≡ ofNat m m<sm → False
  intoSmallerLemm {.(succ _)} {m} {n<m} fzero pr with intoSmaller (canRemoveSuccFrom<N n<m)
  intoSmallerLemm {.(succ _)} {zero} {n<m} fzero refl | bl = zeroNeverGreater (canRemoveSuccFrom<N n<m)
  intoSmallerLemm {.(succ _)} {succ m} {n<m} fzero () | bl
  intoSmallerLemm {.(succ _)} {m} {n<m} (fsucc b) pr with inspect (intoSmaller (canRemoveSuccFrom<N n<m))
  intoSmallerLemm {.(succ _)} {zero} {n<m} (fsucc b) pr | bl with≡ _ = zeroNeverGreater (canRemoveSuccFrom<N n<m)
  intoSmallerLemm {.(succ _)} {succ m} {n<m} {m<sm} (fsucc b) pr | bl with≡ p = intoSmallerLemm {m<sm = canRemoveSuccFrom<N m<sm} b (fsuccInjective pr)

  intoSmallerNotSurj : {n m : ℕ} → {n<m : n <N m} → Surjection (intoSmaller n<m) → False
  intoSmallerNotSurj {n} {zero} {le x ()} record { property = property }
  intoSmallerNotSurj {zero} {succ zero} {n<m} record { property = property } with property fzero
  ... | () , _
  intoSmallerNotSurj {succ n} {succ zero} {n<m} record { property = property } = zeroNeverGreater (canRemoveSuccFrom<N n<m)
  intoSmallerNotSurj {0} {succ (succ m)} {n<m} record { property = property } with property fzero
  ... | () , _
  intoSmallerNotSurj {succ n} {succ (succ m)} {n<m} record { property = property } = problem
    where
      notHit : FinSet (succ (succ m))
      notHit = ofNat (succ m) (le zero refl)
      hitting : Sg (FinSet (succ n)) (λ i → intoSmaller n<m i ≡ notHit)
      hitting = property notHit
      problem : False
      problem with hitting
      ... | a , pr = intoSmallerLemm {succ n} {succ m} {n<m} {le 0 refl} a pr

  finsetDecidableEquality : {n : ℕ} → (x y : FinSet n) → (x ≡ y) || ((x ≡ y) → False)
  finsetDecidableEquality fzero fzero = inl refl
  finsetDecidableEquality fzero (fsucc y) = inr λ ()
  finsetDecidableEquality (fsucc x) fzero = inr λ ()
  finsetDecidableEquality (fsucc x) (fsucc y) with finsetDecidableEquality x y
  finsetDecidableEquality (fsucc x) (fsucc y) | inl pr rewrite pr = inl refl
  finsetDecidableEquality (fsucc x) (fsucc y) | inr pr = inr (λ f → pr (fsuccInjective f))

  subInjection : {n : ℕ} → {f : FinSet (succ (succ n)) → FinSet (succ (succ n))} → Injection f → (FinSet (succ n) → FinSet (succ n))
  subInjection {n} {f} inj x with inspect (f (fsucc x))
  subInjection {f = f} inj x | fzero with≡ _ with inspect (f fzero)
  subInjection {f = f} inj x | fzero with≡ pr | fzero with≡ pr2 = exFalso (fzeroNotFsucc (Injection.property inj (transitivity pr2 (equalityCommutative pr))))
  subInjection {f = f} inj x | fzero with≡ _ | fsucc bl with≡ _ = bl
  subInjection {f = f} inj x | fsucc bl with≡ _ = bl

  subInjIsInjective : {n : ℕ} → {f : FinSet (succ (succ n)) → FinSet (succ (succ n))} → (inj : Injection f) → Injection (subInjection inj)
  Injection.property (subInjIsInjective {f = f} inj) {x} {y} pr with inspect (f (fsucc x))
  Injection.property (subInjIsInjective {f = f} inj) {x} {y} pr | fzero with≡ _ with inspect (f (fzero))
  Injection.property (subInjIsInjective {f = f} inj) {x} {y} pr | fzero with≡ pr1 | fzero with≡ pr2 = exFalso (fzeroNotFsucc (Injection.property inj (transitivity pr2 (equalityCommutative pr1))))
  Injection.property (subInjIsInjective {f = f} inj) {x} {y} pr | fzero with≡ pr1 | fsucc bl with≡ _ with inspect (f (fsucc y))
  Injection.property (subInjIsInjective {f = f} inj) {x} {y} pr | fzero with≡ pr1 | fsucc bl with≡ _ | fzero with≡ x₁ with inspect (f (fzero))
  Injection.property (subInjIsInjective {f = f} inj) {x} {y} pr | fzero with≡ pr1 | fsucc bl with≡ _ | fzero with≡ x1 | fzero with≡ x2 = exFalso (fzeroNotFsucc (Injection.property inj (transitivity x2 (equalityCommutative x1))))
  Injection.property (subInjIsInjective {f = f} inj) {x} {y} refl | fzero with≡ pr1 | fsucc .bl2 with≡ _ | fzero with≡ x1 | fsucc bl2 with≡ _ = fsuccInjective (Injection.property inj (transitivity pr1 (equalityCommutative x1)))
  Injection.property (subInjIsInjective {f = f} inj) {x} {y} refl | fzero with≡ pr1 | fsucc .bl2 with≡ pr2 | fsucc bl2 with≡ pr3 = exFalso (fzeroNotFsucc (Injection.property inj (transitivity pr2 (equalityCommutative pr3))))
  Injection.property (subInjIsInjective {f = f} inj) {x} {y} pr | fsucc bl with≡ _ with inspect (f (fsucc y))
  Injection.property (subInjIsInjective {f = f} inj) {x} {y} pr | fsucc bl with≡ _ | fzero with≡ x₁ with inspect (f fzero)
  Injection.property (subInjIsInjective {f = f} inj) {x} {y} pr | fsucc bl with≡ _ | fzero with≡ x1 | fzero with≡ x2 = exFalso (fzeroNotFsucc (Injection.property inj (transitivity x2 (equalityCommutative x1))))
  Injection.property (subInjIsInjective {f = f} inj) {x} {y} refl | fsucc .bl2 with≡ x1 | fzero with≡ x₁ | fsucc bl2 with≡ x2 = exFalso (fzeroNotFsucc (Injection.property inj (transitivity x2 (equalityCommutative x1))))
  Injection.property (subInjIsInjective {f = f} inj) {x} {y} refl | fsucc .y1 with≡ pr1 | fsucc y1 with≡ pr2 = fsuccInjective (Injection.property inj (transitivity pr1 (equalityCommutative pr2)))

  onepointBij : (f : FinSet 1 → FinSet 1) → Bijection f
  Injection.property (Bijection.inj (onepointBij f)) {x} {y} _ = finset1OnlyOne x y
  Surjection.property (Bijection.surj (onepointBij f)) fzero with inspect (f fzero)
  ... | fzero with≡ pr = fzero , pr
  ... | fsucc () with≡ _
  Surjection.property (Bijection.surj (onepointBij f)) (fsucc ())

  nopointBij : (f : FinSet 0 → FinSet 0) → Bijection f
  Injection.property (Bijection.inj (nopointBij f)) {()}
  Surjection.property (Bijection.surj (nopointBij f)) ()

  flip : {n : ℕ} → (f : FinSet (succ n) → FinSet (succ n)) → FinSet (succ n) → FinSet (succ n)
  flip {n} f fzero = f fzero
  flip {n} f (fsucc a) with inspect (f fzero)
  flip {n} f (fsucc a) | fzero with≡ x = fsucc a
  flip {n} f (fsucc a) | fsucc y with≡ x with finsetDecidableEquality a y
  flip {n} f (fsucc a) | fsucc y with≡ x | inl a=y = fzero
  flip {n} f (fsucc a) | fsucc y with≡ x | inr a!=y = fsucc a

  flipSwapsF0 : {n : ℕ} → {f : FinSet (succ n) → FinSet (succ n)} → (flip f (f fzero)) ≡ fzero
  flipSwapsF0 {zero} {f} with inspect (f fzero)
  flipSwapsF0 {zero} {f} | fzero with≡ x rewrite x = x
  flipSwapsF0 {zero} {f} | fsucc () with≡ x
  flipSwapsF0 {succ n} {f = f} with inspect (f fzero)
  flipSwapsF0 {succ n} {f = f} | fzero with≡ pr rewrite pr = pr
  flipSwapsF0 {succ n} {f = f} | fsucc b with≡ pr rewrite pr = ans
    where
      ans : flip f (fsucc b) ≡ fzero
      ans with inspect (f fzero)
      ans | fzero with≡ x = exFalso (fzeroNotFsucc (transitivity (equalityCommutative x) pr))
      ans | fsucc y with≡ x with finsetDecidableEquality b y
      ans | fsucc y with≡ x | inl b=y = refl
      ans | fsucc y with≡ x | inr b!=y = exFalso (b!=y (fsuccInjective (transitivity (equalityCommutative pr) x)))

  flipSwapsZero : {n : ℕ} → {f : FinSet (succ n) → FinSet (succ n)} → (flip f fzero) ≡ f fzero
  flipSwapsZero {n} {f} = refl

  flipMaintainsEverythingElse : {n : ℕ} → {f : FinSet (succ n) → FinSet (succ n)} → (x : FinSet n) → ((fsucc x ≡ f fzero) → False) → flip f (fsucc x) ≡ fsucc x
  flipMaintainsEverythingElse {n} {f} x x!=f0 with inspect (f fzero)
  flipMaintainsEverythingElse {n} {f} x x!=f0 | fzero with≡ pr = refl
  flipMaintainsEverythingElse {n} {f} x x!=f0 | fsucc bl with≡ pr with finsetDecidableEquality x bl
  flipMaintainsEverythingElse {n} {f} x x!=f0 | fsucc bl with≡ pr | inl x=bl = exFalso (x!=f0 (transitivity (applyEquality fsucc x=bl) (equalityCommutative pr)))
  flipMaintainsEverythingElse {n} {f} x x!=f0 | fsucc bl with≡ pr | inr x!=bl = refl

  bijFlip : {n : ℕ} → {f : FinSet (succ n) → FinSet (succ n)} → Bijection (flip f)
  bijFlip {zero} {f} = onepointBij (flip f)
  Injection.property (Bijection.inj (bijFlip {succ n} {f})) {fzero} {fzero} flx=fly = refl
  Injection.property (Bijection.inj (bijFlip {succ n} {f})) {fzero} {fsucc y} flx=fly with inspect (f fzero)
  Injection.property (Bijection.inj (bijFlip {succ n} {f})) {fzero} {fsucc y} flx=fly | fzero with≡ x rewrite x = exFalso (fzeroNotFsucc flx=fly)
  Injection.property (Bijection.inj (bijFlip {succ n} {f})) {fzero} {fsucc y} flx=fly | fsucc f0 with≡ x with finsetDecidableEquality y f0
  Injection.property (Bijection.inj (bijFlip {succ n} {f})) {fzero} {fsucc y} flx=fly | fsucc f0 with≡ x | inl x₁ = exFalso (fzeroNotFsucc (transitivity (equalityCommutative flx=fly) x))
  Injection.property (Bijection.inj (bijFlip {succ n} {f})) {fzero} {fsucc y} flx=fly | fsucc f0 with≡ x | inr pr = exFalso (pr (fsuccInjective (transitivity (equalityCommutative flx=fly) x)))
  Injection.property (Bijection.inj (bijFlip {succ n} {f})) {fsucc x} {fzero} flx=fly with inspect (f fzero)
  Injection.property (Bijection.inj (bijFlip {succ n} {f})) {fsucc x} {fzero} flx=fly | fzero with≡ pr = transitivity flx=fly pr
  Injection.property (Bijection.inj (bijFlip {succ n} {f})) {fsucc x} {fzero} flx=fly | fsucc f0 with≡ x₁ with finsetDecidableEquality x f0
  Injection.property (Bijection.inj (bijFlip {succ n} {f})) {fsucc x} {fzero} flx=fly | fsucc f0 with≡ pr | inl x₂ = exFalso (fzeroNotFsucc (transitivity flx=fly pr))
  Injection.property (Bijection.inj (bijFlip {succ n} {f})) {fsucc x} {fzero} flx=fly | fsucc f0 with≡ x1 | inr x!=f0 = exFalso (x!=f0 (fsuccInjective (transitivity flx=fly x1)))
  Injection.property (Bijection.inj (bijFlip {succ n} {f})) {fsucc x} {fsucc y} flx=fly with inspect (f fzero)
  Injection.property (Bijection.inj (bijFlip {succ n} {f})) {fsucc x} {fsucc y} flx=fly | fzero with≡ x₁ = flx=fly
  Injection.property (Bijection.inj (bijFlip {succ n} {f})) {fsucc x} {fsucc y} flx=fly | fsucc f0 with≡ x₁ with finsetDecidableEquality y f0
  Injection.property (Bijection.inj (bijFlip {succ n} {f})) {fsucc x} {fsucc y} flx=fly | fsucc f0 with≡ x₁ | inl y=f0 with finsetDecidableEquality x f0
  Injection.property (Bijection.inj (bijFlip {succ n} {f})) {fsucc x} {fsucc y} flx=fly | fsucc f0 with≡ x₁ | inl y=f0 | inl x=f0 = applyEquality fsucc (transitivity x=f0 (equalityCommutative y=f0))
  Injection.property (Bijection.inj (bijFlip {succ n} {f})) {fsucc x} {fsucc y} () | fsucc f0 with≡ x₁ | inl y=f0 | inr x!=f0
  Injection.property (Bijection.inj (bijFlip {succ n} {f})) {fsucc x} {fsucc y} flx=fly | fsucc f0 with≡ x₁ | inr y!=f0 with finsetDecidableEquality x f0
  Injection.property (Bijection.inj (bijFlip {succ n} {f})) {fsucc x} {fsucc y} () | fsucc f0 with≡ x₁ | inr y!=f0 | inl x=f0
  Injection.property (Bijection.inj (bijFlip {succ n} {f})) {fsucc x} {fsucc y} flx=fly | fsucc f0 with≡ x₁ | inr y!=f0 | inr x!=f0 = flx=fly
  Surjection.property (Bijection.surj (bijFlip {succ n} {f})) fzero = f fzero , flipSwapsF0 {f = f}
  Surjection.property (Bijection.surj (bijFlip {succ n} {f})) (fsucc b) with inspect (f fzero)
  Surjection.property (Bijection.surj (bijFlip {succ n} {f})) (fsucc b) | fzero with≡ x = fsucc b , flipMaintainsEverythingElse {f = f} b λ pr → fzeroNotFsucc (transitivity (equalityCommutative x) (equalityCommutative pr))
  Surjection.property (Bijection.surj (bijFlip {succ n} {f})) (fsucc b) | fsucc y with≡ x with finsetDecidableEquality y b
  Surjection.property (Bijection.surj (bijFlip {succ n} {f})) (fsucc b) | fsucc y with≡ x | inl y=b = fzero , transitivity x (applyEquality fsucc y=b)
  Surjection.property (Bijection.surj (bijFlip {succ n} {f})) (fsucc b) | fsucc y with≡ x | inr y!=b = fsucc b , flipMaintainsEverythingElse {f = f} b λ pr → y!=b (fsuccInjective (transitivity (equalityCommutative x) (equalityCommutative pr)))

  injectionIsBijectionFinset : {n : ℕ} → {f : FinSet n → FinSet n} → Injection f → Bijection f
  injectionIsBijectionFinset {zero} {f} inj = record { inj = inj ; surj = record { property = λ () } }
  injectionIsBijectionFinset {succ zero} {f} inj = record { inj = inj ; surj = record { property = ans } }
    where
      ans : (b : FinSet (succ zero)) → Sg (FinSet (succ zero)) (λ a → f a ≡ b)
      ans fzero with inspect (f fzero)
      ans fzero | fzero with≡ x = fzero , x
      ans fzero | fsucc () with≡ x
      ans (fsucc ())
  injectionIsBijectionFinset {succ (succ n)} {f} inj with injectionIsBijectionFinset {succ n} (subInjIsInjective inj)
  ... | sb = ans
    where
      subSurj : Surjection (subInjection inj)
      subSurj = Bijection.surj sb

      tweakedF : FinSet (succ (succ n)) → FinSet (succ (succ n))
      tweakedF fzero = fzero
      tweakedF (fsucc x) = fsucc (subInjection inj x)

      tweakedBij : Bijection tweakedF
      Injection.property (Bijection.inj tweakedBij) {fzero} {fzero} f'x=f'y = refl
      Injection.property (Bijection.inj tweakedBij) {fzero} {fsucc y} ()
      Injection.property (Bijection.inj tweakedBij) {fsucc x} {fzero} ()
      Injection.property (Bijection.inj tweakedBij) {fsucc x} {fsucc y} f'x=f'y = applyEquality fsucc (Injection.property (subInjIsInjective inj) (fsuccInjective f'x=f'y))
      Surjection.property (Bijection.surj tweakedBij) fzero = fzero , refl
      Surjection.property (Bijection.surj tweakedBij) (fsucc b) with Surjection.property subSurj b
      ... | ans , prop = fsucc ans , applyEquality fsucc prop

      compBij : Bijection (λ i → flip f (tweakedF i))
      compBij = bijectionComp tweakedBij bijFlip

      undoTweakMakesF : {x : FinSet (succ (succ n))} → flip f (tweakedF x) ≡ f x
      undoTweakMakesF {fzero} = refl
      undoTweakMakesF {fsucc x} with inspect (f fzero)
      undoTweakMakesF {fsucc x} | fzero with≡ x₁ with inspect (f (fsucc x))
      undoTweakMakesF {fsucc x} | fzero with≡ x₁ | fzero with≡ x₂ with inspect (f fzero)
      undoTweakMakesF {fsucc x} | fzero with≡ x₁ | fzero with≡ x2 | fzero with≡ x3 = exFalso (fzeroNotFsucc (Injection.property inj (transitivity x3 (equalityCommutative x2))))
      undoTweakMakesF {fsucc x} | fzero with≡ x1 | fzero with≡ x2 | fsucc y with≡ x3 = exFalso (fzeroNotFsucc (Injection.property inj (transitivity x1 (equalityCommutative x2))))
      undoTweakMakesF {fsucc x} | fzero with≡ x1 | fsucc y with≡ x2 = equalityCommutative x2
      undoTweakMakesF {fsucc x} | fsucc y with≡ x₁ with inspect (f (fsucc x))
      undoTweakMakesF {fsucc x} | fsucc y with≡ x₁ | fzero with≡ x₂ with inspect (f fzero)
      undoTweakMakesF {fsucc x} | fsucc y with≡ x₁ | fzero with≡ x2 | fzero with≡ x3 = exFalso (fzeroNotFsucc (Injection.property inj (transitivity x3 (equalityCommutative x2))))
      undoTweakMakesF {fsucc x} | fsucc y with≡ x₁ | fzero with≡ x₂ | fsucc pr with≡ x₃ with finsetDecidableEquality pr y
      undoTweakMakesF {fsucc x} | fsucc y with≡ x₁ | fzero with≡ x2 | fsucc pr with≡ x₃ | inl x₄ = equalityCommutative x2
      undoTweakMakesF {fsucc x} | fsucc y with≡ x1 | fzero with≡ x₂ | fsucc pr with≡ x3 | inr pr!=y = exFalso (pr!=y (fsuccInjective (transitivity (equalityCommutative x3) x1)))
      undoTweakMakesF {fsucc x} | fsucc y with≡ x₁ | fsucc thi with≡ x₂ with finsetDecidableEquality thi y
      undoTweakMakesF {fsucc x} | fsucc .thi with≡ x1 | fsucc thi with≡ x2 | inl refl = exFalso false
        where
          p : f fzero ≡ f (fsucc x)
          p = transitivity x1 (equalityCommutative x2)
          q : fzero ≡ fsucc x
          q = Injection.property inj p
          false : False
          false = fzeroNotFsucc q
      undoTweakMakesF {fsucc x} | fsucc y with≡ x₁ | fsucc thi with≡ x2 | inr thi!=y = equalityCommutative x2

      ans : Bijection f
      ans = bijectionPreservedUnderExtensionalEq compBij undoTweakMakesF

  pigeonhole : {n m : ℕ} → (m <N n) → {f : FinSet n → FinSet m} → Injection f → False
  pigeonhole {zero} {zero} () {f} record { property = property }
  pigeonhole {zero} {succ m} () {f} record { property = property }
  pigeonhole {succ n} {zero} m<n {f} record { property = property } = finset0Empty (f fzero)
  pigeonhole {succ zero} {succ m} m<n {f} record { property = property } with canRemoveSuccFrom<N m<n
  pigeonhole {succ zero} {succ m} m<n {f} record { property = property } | le x ()
  pigeonhole {succ (succ n)} {succ zero} m<n {f} record { property = property } = fzeroNotFsucc (property (transitivity f0 (equalityCommutative f1)))
    where
      f0 : f (fzero) ≡ fzero
      f0 with inspect (f fzero)
      ... | fzero with≡ x = x
      ... | fsucc () with≡ _
      f1 : f (fsucc fzero) ≡ fzero
      f1 with inspect (f (fsucc fzero))
      ... | fzero with≡ x = x
      ... | fsucc () with≡ _
  pigeonhole {succ (succ n)} {succ (succ m)} m<n {f} injF = intoSmallerNotSurj {_}{_} {m<n} surj
    where
      inj : Injection ((intoSmaller m<n) ∘ f)
      inj = injComp injF (intoSmallerInj m<n)
      bij : Bijection ((intoSmaller m<n) ∘ f)
      bij = injectionIsBijectionFinset inj
      surj : Surjection (intoSmaller m<n)
      surj = compSurjLeftSurj (Bijection.surj bij)

  --surjectionIsBijectionFinset : {n : ℕ} → {f : FinSet n → FinSet n} → Surjection f → Bijection f
  --surjectionIsBijectionFinset {zero} {f} surj = nopointBij f
  --surjectionIsBijectionFinset {succ zero} {f} surj = onepointBij f
  --surjectionIsBijectionFinset {succ (succ n)} {f} record { property = property } = {!!}

  record FiniteSet {a : _} (A : Set a) : Set a where
    field
      size : ℕ
      mapping : FinSet size → A
      bij : Bijection mapping

  record InfiniteSet {a : _} (A : Set a) : Set a where
    field
      isInfinite : (n : ℕ) → (f : FinSet n → A) → (Bijection f) → False

  finsetInjectIntoℕ : {n : ℕ} → Injection (toNat {n})
  Injection.property (finsetInjectIntoℕ {zero}) {()}
  finsetInjectIntoℕ {succ n} = record { property = ans }
    where
      ans : {n : ℕ} → {x y : FinSet (succ n)} → (toNat x ≡ toNat y) → x ≡ y
      ans {zero} {fzero} {fzero} _ = refl
      ans {zero} {fzero} {fsucc ()}
      ans {zero} {fsucc ()} {y}
      ans {succ n} {fzero} {fzero} pr = refl
      ans {succ n} {fzero} {fsucc y} ()
      ans {succ n} {fsucc x} {fzero} ()
      ans {succ n} {fsucc x} {fsucc y} pr with succInjective pr
      ... | pr' = applyEquality fsucc (ans {n} {x} {y} pr')

  ℕIsInfinite : InfiniteSet ℕ
  InfiniteSet.isInfinite ℕIsInfinite n f bij = pigeonhole (le 0 refl) badInj
    where
      inv : ℕ → FinSet n
      inv = Invertible.inverse (bijectionImpliesInvertible bij)
      invInj : Injection inv
      invInj = Bijection.inj (invertibleImpliesBijection (inverseIsInvertible (bijectionImpliesInvertible bij)))
      bumpUp : FinSet n → FinSet (succ n)
      bumpUp = intoSmaller (le 0 refl)
      bumpUpInj : Injection bumpUp
      bumpUpInj = intoSmallerInj (le 0 refl)
      nextInj : Injection (toNat {succ n})
      nextInj = finsetInjectIntoℕ {succ n}
      bad : FinSet (succ n) → FinSet n
      bad a = (inv (toNat a))
      badInj : Injection bad
      badInj = injComp nextInj invInj

  finsetNotInfinite : {n : ℕ} → InfiniteSet (FinSet n) → False
  finsetNotInfinite {n} record { isInfinite = isInfinite } = isInfinite n id idIsBijective