Smaug/agda
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{-# OPTIONS --warning=error --safe #-}

open import LogicalFormulae
open import Naturals
open import WellFoundedInduction
open import KeyValue

module PrimeNumbers where

    record divisionAlgResult (a : ℕ) (b : ℕ) : Set where
      field
        quot : ℕ
        rem : ℕ
        pr : a *N quot +N rem ≡ b
        remIsSmall : (rem <N a) || (a ≡ 0)

    divAlgLessLemma : (a b : ℕ) → (0 <N a) → (r : divisionAlgResult a b) → (divisionAlgResult.quot r ≡ 0) || (divisionAlgResult.rem r <N b)
    divAlgLessLemma zero b pr _ = exFalso (lessIrreflexive pr)
    divAlgLessLemma (succ a) b _ record { quot = zero ; rem = a%b ; pr = pr ; remIsSmall = remIsSmall } = inl refl
    divAlgLessLemma (succ a) b _ record { quot = (succ a/b) ; rem = a%b ; pr = pr ; remIsSmall = remIsSmall } = inr record { x = a/b +N a *N succ (a/b) ; proof = pr }

    modUniqueLemma : {rem1 rem2 a : ℕ} → (quot1 quot2 : ℕ) → rem1 <N a → rem2 <N a → a *N quot1 +N rem1 ≡ a *N quot2 +N rem2 → rem1 ≡ rem2
    modUniqueLemma {rem1} {rem2} {a} zero zero rem1<a rem2<a pr rewrite productZeroIsZeroRight a = pr
    modUniqueLemma {rem1} {rem2} {a} zero (succ quot2) rem1<a rem2<a pr rewrite productZeroIsZeroRight a | pr | multiplicationNIsCommutative a (succ quot2) | additionNIsAssociative a (quot2 *N a) rem2 = exFalso (cannotAddAndEnlarge' {a} {quot2 *N a +N rem2} rem1<a)
    modUniqueLemma {rem1} {rem2} {a} (succ quot1) zero rem1<a rem2<a pr rewrite productZeroIsZeroRight a | equalityCommutative pr | multiplicationNIsCommutative a (succ quot1) | additionNIsAssociative a (quot1 *N a) rem1 = exFalso (cannotAddAndEnlarge' {a} {quot1 *N a +N rem1} rem2<a)
    modUniqueLemma {rem1} {rem2} {a} (succ quot1) (succ quot2) rem1<a rem2<a pr rewrite multiplicationNIsCommutative a (succ quot1) | multiplicationNIsCommutative a (succ quot2) | additionNIsAssociative a (quot1 *N a) rem1 | additionNIsAssociative a (quot2 *N a) rem2 = modUniqueLemma {rem1} {rem2} {a} quot1 quot2 rem1<a rem2<a (go {a}{quot1}{rem1}{quot2}{rem2} pr)
      where
        go : {a quot1 rem1 quot2 rem2 : ℕ} → (a +N (quot1 *N a +N rem1) ≡ a +N (quot2 *N a +N rem2)) → a *N quot1 +N rem1 ≡ a *N quot2 +N rem2
        go {a} {quot1} {rem1} {quot2} {rem2} pr rewrite multiplicationNIsCommutative quot1 a | multiplicationNIsCommutative quot2 a = canSubtractFromEqualityLeft {a} pr

    modIsUnique : {a b : ℕ} → (div1 div2 : divisionAlgResult a b) → divisionAlgResult.rem div1 ≡ divisionAlgResult.rem div2
    modIsUnique {zero} {b} record { quot = quot1 ; rem = rem1 ; pr = pr1 ; remIsSmall = remIsSmall1 } record { quot = quot ; rem = rem ; pr = pr ; remIsSmall = remIsSmall } = transitivity pr1 (equalityCommutative pr)
    modIsUnique {succ a} {b} record { quot = quot1 ; rem = rem1 ; pr = pr1 ; remIsSmall = (inl y) } record { quot = quot ; rem = rem ; pr = pr ; remIsSmall = (inl x) } = modUniqueLemma {rem1} {rem} {succ a} quot1 quot y x (transitivity pr1 (equalityCommutative pr))
    modIsUnique {succ a} {b} record { quot = quot1 ; rem = rem1 ; pr = pr1 ; remIsSmall = (inr ()) } record { quot = quot ; rem = rem ; pr = pr ; remIsSmall = (inl x) }
    modIsUnique {succ a} {b} record { quot = quot1 ; rem = rem1 ; pr = pr1 ; remIsSmall = remIsSmall1 } record { quot = quot ; rem = rem ; pr = pr ; remIsSmall = (inr ()) }

    transferAddition : (a b c : ℕ) → (a +N b) +N c ≡ (a +N c) +N b
    transferAddition a b c rewrite (additionNIsAssociative a b c) = p a b c
      where
        p : (a b c : ℕ) → a +N (b +N c) ≡ (a +N c) +N b
        p a b c rewrite (additionNIsCommutative b c) = equalityCommutative (additionNIsAssociative a c b)

    divisionAlgLemma : (x b : ℕ) → x *N zero +N b ≡ b
    divisionAlgLemma x b rewrite (productZeroIsZeroRight x) = refl

    divisionAlgLemma2 : (x b : ℕ) → (x ≡ b) → x *N succ zero +N zero ≡ b
    divisionAlgLemma2 x b pr rewrite (productWithOneRight x) = equalityCommutative (transitivity (equalityCommutative pr) (equalityCommutative (addZeroRight x)))

    divisionAlgLemma3 : {a x : ℕ} → (p : succ a <N succ x) → (subtractionNResult.result (-N (inl p))) <N (succ x)
    divisionAlgLemma3 {a} {x} p = -NIsDecreasing {a} {succ x} p

    divisionAlgLemma4 : (p a q : ℕ) → ((p +N a *N p) +N q) +N succ a ≡ succ ((p +N a *N succ p) +N q)
    divisionAlgLemma4 p a q = ans
      where
        r   : ((p +N a *N p) +N q) +N succ a ≡ succ (((p +N a *N p) +N q) +N a)
        ans : ((p +N a *N p) +N q) +N succ a ≡ succ ((p +N a *N succ p) +N q)
        s : ((p +N a *N p) +N q) +N a ≡ (p +N a *N succ p) +N q
        t : (p +N a *N p) +N a ≡ p +N a *N succ p
        s = transitivity (transferAddition (p +N a *N p) q a) (applyEquality (λ i → i +N q) t)
        ans = identityOfIndiscernablesRight (((p +N a *N p) +N q) +N succ a) (succ (((p +N a *N p) +N q) +N a)) (succ ((p +N a *N succ p) +N q)) _≡_ r (applyEquality succ s)
        r = succExtracts ((p +N a *N p) +N q) a
        t = transitivity (additionNIsAssociative p (a *N p) a) (applyEquality (λ n → p +N n) (equalityCommutative (aSucB a p)))

    divisionAlg : (a : ℕ) → (b : ℕ) → divisionAlgResult a b
    divisionAlg zero = λ b → record { quot = zero ; rem = b ; pr = refl ; remIsSmall = inr refl }
    divisionAlg (succ a) = rec <NWellfounded (λ n → divisionAlgResult (succ a) n) go
      where
        go : (x : ℕ) (indHyp : (y : ℕ) (y<x : y <N x) → divisionAlgResult (succ a) y) →
               divisionAlgResult (succ a) x
        go zero prop = record { quot = zero ; rem = zero ; pr = divisionAlgLemma (succ a) zero ; remIsSmall = inl (succIsPositive a) }
        go (succ x) indHyp with orderIsTotal (succ a) (succ x)
        go (succ x) indHyp | inl (inl sa<sx) with indHyp (subtractionNResult.result (-N (inl sa<sx))) (divisionAlgLemma3 sa<sx)
        ... | record { quot = prevQuot ; rem = prevRem ; pr = prevPr ; remIsSmall = smallRem} = p
          where
            p : divisionAlgResult (succ a) (succ x)
            addedA : (succ a *N prevQuot +N prevRem) +N (succ a) ≡ subtractionNResult.result (-N (inl sa<sx)) +N (succ a)
            addedA' : (succ a *N prevQuot +N prevRem) +N succ a ≡ succ x
            addedA'' : (succ a *N succ prevQuot) +N prevRem ≡ succ x
            addedA''' : succ ((prevQuot +N a *N succ prevQuot) +N prevRem) ≡ succ x
            addedA''' = identityOfIndiscernablesLeft ((succ a *N succ prevQuot) +N prevRem) (succ x) (succ ((prevQuot +N a *N succ prevQuot) +N prevRem)) _≡_ addedA'' refl
            p = record { quot = succ prevQuot ; rem = prevRem ; pr = addedA''' ; remIsSmall = smallRem}
            addedA = applyEquality (λ n → n +N succ a) prevPr
            addedA' = identityOfIndiscernablesRight ((succ a *N prevQuot +N prevRem) +N succ a) (subtractionNResult.result (-N (inl sa<sx)) +N (succ a)) (succ x) _≡_ addedA (addMinus {succ a} {succ x} (inl sa<sx))
            addedA'' = identityOfIndiscernablesLeft ((succ a *N prevQuot +N prevRem) +N succ a) (succ x) ((succ a *N succ prevQuot) +N prevRem) _≡_ addedA' (divisionAlgLemma4 prevQuot a prevRem)
        go (succ x) indHyp | inr (sa=sx) = record { quot = succ zero ; rem = zero ; pr = divisionAlgLemma2 (succ a) (succ x) sa=sx ; remIsSmall = inl (succIsPositive a)}
        go (succ x) indHyp | inl (inr (sx<sa)) = record { quot = zero ; rem = succ x ; pr = divisionAlgLemma (succ a) (succ x) ; remIsSmall = inl sx<sa }

    data _∣_ : ℕ → ℕ → Set where
      divides : {a b : ℕ} → (res : divisionAlgResult a b) → divisionAlgResult.rem res ≡ zero → a ∣ b

    zeroDividesNothing : (a : ℕ) → zero ∣ succ a → False
    zeroDividesNothing a (divides record { quot = quot ; rem = rem ; pr = pr } x) = naughtE p
      where
        p : zero ≡ succ a
        p = transitivity (equalityCommutative x) pr

    twoDividesFour : succ (succ zero) ∣ succ (succ (succ (succ zero)))
    twoDividesFour = divides {(succ (succ zero))} {succ (succ (succ (succ zero)))} (record { quot = succ (succ zero) ; rem = zero ; pr = refl ; remIsSmall = inl (succIsPositive 1)}) refl

    record Prime (p : ℕ) : Set where
      field
        p>1 : 1 <N p
        pr : forall {i : ℕ} → i ∣ p → i <N p → zero <N i → i ≡ (succ zero)

    record Composite (n : ℕ) : Set where
      field
        n>1 : 1 <N n
        divisor : ℕ
        dividesN : divisor ∣ n
        divisorLessN : divisor <N n
        divisorNot1 : 1 <N divisor
        divisorPrime : Prime divisor
        noSmallerDivisors : ∀ i → i <N divisor → 1 <N i → i ∣ n → False

    notBothPrimeAndComposite : {n : ℕ} → Composite n → Prime n → False
    notBothPrimeAndComposite {n} record { n>1 = n>1 ; divisor = divisor ; dividesN = dividesN ; divisorLessN = divisorLessN ; divisorNot1 = divisorNot1 } record { p>1 = p>1 ; pr = pr } = lessImpliesNotEqual divisorNot1 (equalityCommutative div=1)
      where
        div=1 : divisor ≡ 1
        div=1 = pr {divisor} dividesN divisorLessN (orderIsTransitive (succIsPositive 0) divisorNot1)

    zeroIsNotPrime : Prime 0 → False
    zeroIsNotPrime record { p>1 = p>1 ; pr = pr } = zeroNeverGreater p>1

    oneIsNotPrime : Prime 1 → False
    oneIsNotPrime record { p>1 = (le x proof) ; pr = pr } = naughtE (equalityCommutative absurd')
      where
        absurd : x +N 1 ≡ 0
        absurd = succInjective proof
        absurd' : succ x ≡ 0
        absurd' rewrite additionNIsCommutative 1 x = absurd

    twoIsPrime : Prime 2
    Prime.p>1 twoIsPrime = succPreservesInequality (succIsPositive 0)
    Prime.pr twoIsPrime {i} i|2 i<2 0<i with orderIsTotal i (succ (succ zero))
    Prime.pr twoIsPrime {zero} i|2 i<2 (le x ()) | order
    Prime.pr twoIsPrime {succ zero} i|2 i<2 0<i | order = refl
    Prime.pr twoIsPrime {succ (succ zero)} i|2 i<2 0<i | order = exFalso (lessImpliesNotEqual {2} i<2 refl)
    Prime.pr twoIsPrime {succ (succ (succ i))} i|2 i<2 0<i | inl (inl x) = exFalso (orderIsIrreflexive i<2 (succPreservesInequality (succPreservesInequality (succIsPositive i))))
    Prime.pr twoIsPrime {succ (succ (succ i))} i|2 i<2 0<i | inl (inr twoLessThree) = exFalso (orderIsIrreflexive twoLessThree i<2)
    Prime.pr twoIsPrime {succ (succ (succ i))} i|2 i<2 0<i | inr ()

    compositeImpliesNotPrime : (m p : ℕ) → (succ zero <N m) → (m <N p) → (m ∣ p) → Prime p → False
    compositeImpliesNotPrime zero p (le x ()) _ mDivP pPrime
    compositeImpliesNotPrime (succ zero) p mLarge _ mDivP pPrime = lessImpliesNotEqual {succ zero} {succ zero} mLarge refl
    compositeImpliesNotPrime (succ (succ m)) zero _ _ mDivP ()
    compositeImpliesNotPrime (succ (succ m)) (succ zero) _ _ mDivP primeP = exFalso (oneIsNotPrime primeP)
    compositeImpliesNotPrime (succ (succ m)) (succ (succ p)) _ mLessP mDivP pPrime = false
      where
        r = succ (succ m)
        q = succ (succ p)
        rEqOne : r ≡ succ zero
        rEqOne = (Prime.pr pPrime) {r} mDivP mLessP (succIsPositive (succ m))
        false : False
        false = succIsNonzero (succInjective rEqOne)

    fourIsNotPrime : Prime 4 → False
    fourIsNotPrime = compositeImpliesNotPrime (succ (succ zero)) (succ (succ (succ (succ zero)))) (le zero refl) (le (succ zero) refl) twoDividesFour

    record hcfData (a b : ℕ) : Set where
      field
        c : ℕ
        c|a : c ∣ a
        c|b : c ∣ b
        hcf : ∀ x → x ∣ a → x ∣ b → x ∣ c

    record extensionalHCF (a b : ℕ) (a<=b : a ≤N b) : Set where
      field
        c : ℕ
        c|a : c ∣ a
        c|b : c ∣ b
        hcfExtension : Map () () ℕTotalOrder

    hcfsEquivalent : {a b : ℕ} → hcfData a b → extensionalHCF a b
    hcfsEquivalent {a} {b} record { c = c ; c|a = c|a ; c|b = c|b ; hcf = hcf } = record { c = c ; c|a = c|a ; c|b = c|b ; hcfExtension = ? }

    record extendedHcf (a b : ℕ) : Set where
      field
        hcf : hcfData a b
      c : ℕ
      c = hcfData.c hcf
      field
        extended1 : ℕ
        extended2 : ℕ
        extendedProof : (a *N extended1 ≡ b *N extended2 +N c) || (a *N extended1 +N c ≡ b *N extended2)

    divEqualityLemma1 : {a b c : ℕ} → b ≡ zero → b *N c +N 0 ≡ a → a ≡ b
    divEqualityLemma1 {a} {.0} {c} refl pr = equalityCommutative pr

    divEquality : {a b : ℕ} → a ∣ b → b ∣ a → a ≡ b
    divEquality {zero} {b} (divides record { quot = quot ; rem = rem ; pr = pr } x) prBA = transitivity (equalityCommutative x) pr
    divEquality {succ a'} {zero} prAB (divides record { quot = quot ; rem = rem ; pr = pr } x) = transitivity (equalityCommutative pr) x
    divEquality {succ a'} {succ b'} (divides record { quot = aDivB ; rem = .0 ; pr = prAB } refl) (divides record { quot = bDivA ; rem = .0 ; pr = prBA } refl) = p v
      where
        a = succ a'
        b = succ b'
        q : b *N bDivA ≡ a
        r : a *N aDivB ≡ b
        s : (b *N bDivA) *N aDivB ≡ b
        t : b *N (bDivA *N aDivB) ≡ b
        t' : b *N (bDivA *N aDivB) ≡ b *N 1
        u : (b ≡ zero) || (bDivA *N aDivB ≡ 1)
        u' : ((b ≡ zero) || (bDivA *N aDivB ≡ 1)) → bDivA *N aDivB ≡ 1
        u'' : (bDivA *N aDivB ≡ 1)
        v : (bDivA ≡ 1) && (aDivB ≡ 1)
        v = productOneImpliesOperandsOne u''
        p : ((bDivA ≡ 1) && (aDivB ≡ 1)) → a ≡ b
        p record { fst = bDivA=1 ; snd = aDivB=1 } = go prAB
          where
            go : (aDivB +N a' *N aDivB) +N 0 ≡ succ b' → a ≡ b
            go pr rewrite aDivB=1 = go' pr
              where
                go' : succ (a' *N 1 +N 0) ≡ succ b' → a ≡ b
                go' pr rewrite equalityCommutative (oneTimesPlusZero a') = pr
        u' (inl x) = exFalso ((succIsNonzero {b'}) x)
        u' (inr x) = x
        u'' = u' u
        u = productCancelsLeft' b (bDivA *N aDivB) 1 t'
        t' = identityOfIndiscernablesRight (b *N (bDivA *N aDivB)) b (b *N 1) _≡_ t (equalityCommutative (productWithOneRight b))
        t = identityOfIndiscernablesLeft ((b *N bDivA) *N aDivB) b (b *N (bDivA *N aDivB)) _≡_ s (equalityCommutative (multiplicationNIsAssociative b bDivA aDivB))
        s = identityOfIndiscernablesLeft (a *N aDivB) b ((b *N bDivA) *N aDivB) _≡_ r (equalityCommutative (applyEquality (λ i → i *N aDivB) q))
        r = identityOfIndiscernablesLeft (a *N aDivB +N 0) b (a *N aDivB) _≡_ prAB (addZeroRight (a *N aDivB))
        q = identityOfIndiscernablesLeft (b *N bDivA +N 0) a (b *N bDivA) _≡_ prBA (addZeroRight (b *N bDivA))

    hcfWelldefined : {a b : ℕ} → (ab : hcfData a b) → (ab' : hcfData a b) → (hcfData.c ab ≡ hcfData.c ab')
    hcfWelldefined {a} {b} record { c = c ; c|a = c|a ; c|b = c|b ; hcf = hcf } record { c = c' ; c|a = c|a' ; c|b = c|b' ; hcf = hcf' } with hcf c' c|a' c|b'
    ... | c'DivC with hcf' c c|a c|b
    ... | cDivC' = divEquality cDivC' c'DivC

    reverseHCF : {a b : ℕ} → (ab : extendedHcf a b) → extendedHcf b a
    reverseHCF {a} {b} record { hcf = record { c = c ; c|a = c|a ; c|b = c|b ; hcf = hcf } ; extended1 = extended1 ; extended2 = extended2 ; extendedProof = (inl x) } = record { hcf = record { c = c ; c|a = c|b ; c|b = c|a ; hcf = λ x z z₁ → hcf x z₁ z } ; extended1 = extended2 ; extended2 = extended1 ; extendedProof = inr (equalityCommutative x) }
    reverseHCF {a} {b} record { hcf = record { c = c ; c|a = c|a ; c|b = c|b ; hcf = hcf } ; extended1 = extended1 ; extended2 = extended2 ; extendedProof = (inr x) } = record { hcf = record { c = c ; c|a = c|b ; c|b = c|a ; hcf = λ x z z₁ → hcf x z₁ z } ; extended1 = extended2 ; extended2 = extended1 ; extendedProof = inl (equalityCommutative x) }

    aDivA : (a : ℕ) → a ∣ a
    aDivA zero = divides (record { quot = 1 ; rem = 0 ; pr = equalityCommutative (oneTimesPlusZero zero) ; remIsSmall = inr refl }) refl
    aDivA (succ a) = divides (record { quot = 1 ; rem = 0 ; pr = equalityCommutative (oneTimesPlusZero (succ a)) ; remIsSmall = inl (succIsPositive a) }) refl

    aDivZero : (a : ℕ) → a ∣ zero
    aDivZero zero = aDivA zero
    aDivZero (succ a) = divides (record { quot = zero ; rem = zero ; pr = lemma (succ a) ; remIsSmall = inl (succIsPositive a)}) refl
      where
        lemma : (b : ℕ) → b *N zero +N zero ≡ zero
        lemma b rewrite (addZeroRight (b *N zero)) = productZeroIsZeroRight b

    oneDivN : (a : ℕ) → 1 ∣ a
    oneDivN a = divides (record { quot = a ; rem = zero ; pr = pr ; remIsSmall = inl (succIsPositive zero) }) refl
      where
        pr : (a +N zero) +N zero ≡ a
        pr rewrite addZeroRight (a +N zero) = addZeroRight a

    hcfZero : (a : ℕ) → extendedHcf zero a
    hcfZero a = record { hcf = record { c = a ; c|a = aDivZero a ; c|b = aDivA a ; hcf = λ _ _ p → p } ; extended1 = 0 ; extended2 = 1 ; extendedProof = inr (equalityCommutative (productWithOneRight a))}

    hcfOne : (a : ℕ) → extendedHcf 1 a
    hcfOne a = record { hcf = record { c = 1 ; c|a = aDivA 1 ; c|b = oneDivN a ; hcf = λ _ z _ → z } ; extended1 = 1 ; extended2 = 0 ; extendedProof = inl g }
      where
        g : 1 ≡ a *N 0 +N 1
        g rewrite multiplicationNIsCommutative a 0 = refl

    zeroIsValidRem : (a : ℕ) → (0 <N a) || (a ≡ 0)
    zeroIsValidRem zero = inr refl
    zeroIsValidRem (succ a) = inl (succIsPositive a)

    dividesBothImpliesDividesSum : {a x y : ℕ} → a ∣ x → a ∣ y → a ∣ (x +N y)
    dividesBothImpliesDividesSum {a} {x} {y} (divides record { quot = xDivA ; rem = .0 ; pr = prA } refl) (divides record { quot = quot ; rem = .0 ; pr = pr } refl) = divides (record { quot = xDivA +N quot ; rem = 0 ; pr = go {a} {x} {y} {xDivA} {quot} pr prA ; remIsSmall = zeroIsValidRem a }) refl
      where
        go : {a x y quot quot2 : ℕ} → (a *N quot2 +N zero ≡ y) → (a *N quot +N zero ≡ x) → a *N (quot +N quot2) +N zero ≡ x +N y
        go {a} {x} {y} {quot} {quot2} pr1 pr2 rewrite addZeroRight (a *N quot) = identityOfIndiscernablesLeft (a *N (quot +N quot2)) (x +N y) (a *N (quot +N quot2) +N zero) _≡_ t (equalityCommutative (addZeroRight (a *N (quot +N quot2))))
          where
            t : a *N (quot +N quot2) ≡ x +N y
            t rewrite addZeroRight (a *N quot2) = transitivity (productDistributes a quot quot2) p
              where
                s : a *N quot +N a *N quot2 ≡ x +N a *N quot2
                s = applyEquality (λ n → n +N a *N quot2) pr2
                r : x +N a *N quot2 ≡ x +N y
                r = applyEquality (λ n → x +N n) pr1
                p : a *N quot +N a *N quot2 ≡ x +N y
                p = transitivity s r

    dividesBothImpliesDividesDifference : {a b c : ℕ} → a ∣ b → a ∣ c → (c<b : c <N b) → a ∣ (subtractionNResult.result (-N (inl c<b)))
    dividesBothImpliesDividesDifference {zero} {b} {.0} prab (divides record { quot = quot ; rem = .0 ; pr = refl } refl) c<b = prab
    dividesBothImpliesDividesDifference {succ a} {b} {c} (divides record { quot = bDivSA ; rem = .0 ; pr = pr } refl) (divides record { quot = cDivSA ; rem = .0 ; pr = pr2 } refl) c<b rewrite (addZeroRight (succ a *N cDivSA)) | (addZeroRight (succ a *N bDivSA)) = divides (record { quot = subtractionNResult.result bDivSA-cDivSA ; rem = 0 ; pr = identityOfIndiscernablesLeft (succ a *N (subtractionNResult.result bDivSA-cDivSA)) (subtractionNResult.result (-N (inl c<b))) (succ a *N (subtractionNResult.result bDivSA-cDivSA) +N zero) _≡_ (identityOfIndiscernablesLeft (la-ka) (subtractionNResult.result (-N (inl c<b))) (succ a *N (subtractionNResult.result bDivSA-cDivSA)) _≡_ s (equalityCommutative q)) (equalityCommutative (addZeroRight _)) ; remIsSmall = inl (succIsPositive a)}) refl
      where
        p : cDivSA <N bDivSA
        p rewrite (equalityCommutative pr2) | (equalityCommutative pr) = cancelInequalityLeft {succ a} {cDivSA} {bDivSA} c<b
        bDivSA-cDivSA : subtractionNResult cDivSA bDivSA (inl p)
        bDivSA-cDivSA = -N {cDivSA} {bDivSA} (inl p)
        la-ka = subtractionNResult.result (-N {succ a *N cDivSA} {succ a *N bDivSA} (inl (lessRespectsMultiplicationLeft cDivSA bDivSA (succ a) p (succIsPositive a))))
        q : (succ a *N (subtractionNResult.result bDivSA-cDivSA)) ≡ la-ka
        q = subtractProduct {succ a} {cDivSA} {bDivSA} (succIsPositive a) p
        s : la-ka ≡ subtractionNResult.result (-N {c} {b} (inl c<b))
        s = equivalentSubtraction (succ a *N cDivSA) b (succ a *N bDivSA) c (lessRespectsMultiplicationLeft cDivSA bDivSA (succ a) p (succIsPositive a)) c<b g
          where
            g : (succ a *N cDivSA) +N b ≡ (succ a *N bDivSA) +N c
            g rewrite equalityCommutative pr2 | equalityCommutative pr = additionNIsCommutative (cDivSA +N a *N cDivSA) (bDivSA +N a *N bDivSA)

    euclidLemma1 : {a b : ℕ} → (a<b : a <N b) → (t : ℕ) → a +N b <N t → a +N (subtractionNResult.result (-N (inl a<b))) <N t
    euclidLemma1 {zero} {b} zero<b t b<t = b<t
    euclidLemma1 {succ a} {b} sa<b t sa+b<t = identityOfIndiscernablesLeft (subtractionNResult.result (-N (inl sa<b)) +N succ a) t (succ a +N (subtractionNResult.result (-N (inl sa<b)))) _<N_ q (additionNIsCommutative (subtractionNResult.result (-N (inl sa<b))) (succ a))
      where
        p : b <N t
        p = orderIsTransitive (le a refl) sa+b<t
        q : (subtractionNResult.result (-N (inl sa<b))) +N succ a <N t
        q = identityOfIndiscernablesLeft b t (subtractionNResult.result (-N (inl sa<b)) +N succ a) _<N_ p (equalityCommutative (addMinus (inl sa<b)))

    euclidLemma2 : {a b max : ℕ} → (succ (a +N b) <N max) → b <N max
    euclidLemma2 {a} {b} {max} pr = lessTransitive {b} {succ (a +N b)} {max} (lemma a b) pr
      where
        lemma : (a b : ℕ) → b <N succ (a +N b)
        lemma a zero = succIsPositive (a +N zero)
        lemma a (succ b) = succPreservesInequality (collapseSuccOnRight {b} {a} {b} (lemma a b))

    euclidLemma3 : {a b max : ℕ} → (succ (succ (a +N b)) <N max) → succ b <N max
    euclidLemma3 {a} {b} {max} pr = euclidLemma2 {a} {succ b} {max} (identityOfIndiscernablesLeft (succ (succ (a +N b))) max (succ (a +N succ b)) _<N_ pr (applyEquality succ (equalityCommutative (succExtracts a b))))

    euclidLemma4 : (a b c d h : ℕ) → (sa<b : (succ a) <N b) → (pr : subtractionNResult.result (-N (inl sa<b)) *N c ≡ (succ a) *N d +N h) → b *N c ≡ (succ a) *N (d +N c) +N h
    euclidLemma4 a b zero d h sa<b pr rewrite addZeroRight d | productZeroIsZeroRight b | productZeroIsZeroRight (subtractionNResult.result (-N (inl sa<b))) = pr
    euclidLemma4 a b (succ c) d h sa<b pr rewrite subtractProduct' (succIsPositive c) sa<b = transitivity q' r'
      where
        q : (succ c) *N b ≡ succ (a +N c *N succ a) +N ((succ a) *N d +N h)
        q = moveOneSubtraction {succ (a +N c *N succ a)} {b +N c *N b} {(succ a) *N d +N h} {inl _} pr
        q' : b *N succ c ≡ succ (a +N c *N succ a) +N ((succ a) *N d +N h)
        q' rewrite multiplicationNIsCommutative b (succ c) = q
        r' : ((succ c) *N succ a) +N (((succ a) *N d) +N h) ≡ ((succ a) *N (d +N succ c)) +N h
        r' rewrite equalityCommutative (additionNIsAssociative ((succ c) *N succ a) ((succ a) *N d) h) = applyEquality (λ t → t +N h) {((succ c) *N succ a) +N ((succ a) *N d)} {(succ a) *N (d +N succ c)} (go (succ c) (succ a) d)
          where
            go' : (a b c : ℕ) → b *N a +N b *N c ≡ b *N (c +N a)
            go : (a b c : ℕ) → a *N b +N b *N c ≡ b *N (c +N a)
            go a b c rewrite multiplicationNIsCommutative a b = go' a b c
            go' a b c rewrite additionNIsCommutative (b *N a) (b *N c) = equalityCommutative (productDistributes b c a)

    euclidLemma5 : (a b c d h : ℕ) → (sa<b : (succ a) <N b) → (pr : subtractionNResult.result (-N (inl sa<b)) *N c +N h ≡ (succ a) *N d) → (succ a) *N (d +N c) ≡ b *N c +N h
    euclidLemma5 a b c d h sa<b pr with (-N (inl sa<b))
    euclidLemma5 a b zero d h sa<b pr | record { result = result ; pr = sub } rewrite addZeroRight d | productZeroIsZeroRight b | productZeroIsZeroRight result = equalityCommutative pr
    euclidLemma5 a b (succ c) d h sa<b pr | record { result = result ; pr = sub } rewrite subtractProduct' (succIsPositive c) sa<b | equalityCommutative sub = pv''
      where
        p : succ a *N d ≡ result *N succ c +N h
        p = equalityCommutative pr
        p' : a *N succ c +N succ a *N d ≡ (a *N succ c) +N ((result *N succ c) +N h)
        p' = applyEquality (λ t → a *N succ c +N t) p
        p'' : a *N succ c +N succ a *N d ≡ (a *N succ c +N result *N succ c) +N h
        p'' rewrite (additionNIsAssociative (a *N succ c) (result *N succ c) h) = p'
        p''' : a *N succ c +N succ a *N d ≡ (a +N result) *N succ c +N h
        p''' rewrite productDistributes' a result (succ c) = p''
        pv : c +N (a *N succ c +N succ a *N d) ≡ (c +N (a +N result) *N succ c) +N h
        pv rewrite additionNIsAssociative c ((a +N result) *N succ c) h = applyEquality (λ t → c +N t) p'''
        pv' : (succ c) +N (a *N succ c +N succ a *N d) ≡ succ ((c +N (a +N result) *N succ c) +N h)
        pv' = applyEquality succ pv
        pv'' : (succ a) *N (d +N succ c) ≡ succ ((c +N (a +N result) *N succ c) +N h)
        pv'' = identityOfIndiscernablesLeft ((succ c) +N (a *N succ c +N succ a *N d)) _ ((succ a) *N (d +N succ c)) _≡_ pv' (go a c d)
          where
            go : (a c d : ℕ) → (succ c) +N (a *N succ c +N ((succ a) *N d)) ≡ (succ a) *N (d +N succ c)
            go a c d rewrite equalityCommutative (additionNIsAssociative (succ c) (a *N succ c) ((succ a) *N d)) = go'
              where
                go' : (succ a) *N (succ c) +N (succ a) *N d ≡ (succ a) *N (d +N succ c)
                go' rewrite additionNIsCommutative d (succ c) = equalityCommutative (productDistributes (succ a) (succ c) d)

    euclid : (a b : ℕ) → extendedHcf a b
    euclid a b = inducted (succ a +N b) a b (a<SuccA (a +N b))
      where
        P : ℕ → Set
        P sum = ∀ (a b : ℕ) → a +N b <N sum → extendedHcf a b
        go'' : {a b : ℕ} → (maxsum : ℕ) → (a <N b) → (a +N b <N maxsum) → (∀ y → y <N maxsum → P y) → extendedHcf a b
        go'' {zero} {b} maxSum zero<b b<maxsum indHyp = hcfZero b
        go'' {1} {b} maxSum 1<b b<maxsum indHyp = hcfOne b
        go'' {succ (succ a)} {b} maxSum ssa<b ssa+b<maxsum indHyp with (indHyp (succ b) (euclidLemma3 {a} {b} {maxSum} ssa+b<maxsum)) (subtractionNResult.result (-N (inl ssa<b))) (succ (succ a)) (identityOfIndiscernablesLeft b (succ b) (subtractionNResult.result (-N (inl ssa<b)) +N succ (succ a)) _<N_ (a<SuccA b) (equalityCommutative (addMinus (inl ssa<b))))
        go'' {succ (succ a)} {b} maxSum ssa<b ssa+b<maxsum indHyp | record { hcf = record { c = c ; c|a = c|a ; c|b = c|b ; hcf = hcf } ; extended1 = extended1 ; extended2 = extended2 ; extendedProof = inl extendedProof } = record { hcf = record { c = c ; c|a = c|b ; c|b = hcfDivB'' ; hcf = λ div prDivSSA prDivB → hcf div (dividesBothImpliesDividesDifference prDivB prDivSSA ssa<b) prDivSSA } ; extended2 = extended1; extended1 = extended2 +N extended1 ; extendedProof = inr (equalityCommutative (euclidLemma4 (succ a) b extended1 extended2 c ssa<b extendedProof)) }
          where
            hcfDivB : c ∣ ((succ (succ a)) +N (subtractionNResult.result (-N (inl ssa<b))))
            hcfDivB = dividesBothImpliesDividesSum {c} {succ (succ a)} { subtractionNResult.result (-N (inl ssa<b))} c|b c|a
            hcfDivB' : c ∣ ((subtractionNResult.result (-N (inl ssa<b))) +N (succ (succ a)))
            hcfDivB' = identityOfIndiscernablesRight c ((succ (succ a)) +N (subtractionNResult.result (-N (inl ssa<b)))) ((subtractionNResult.result (-N (inl ssa<b))) +N (succ (succ a))) _∣_ hcfDivB (additionNIsCommutative (succ (succ a)) ( subtractionNResult.result (-N (inl ssa<b))))
            hcfDivB'' : c ∣ b
            hcfDivB'' = identityOfIndiscernablesRight c ((subtractionNResult.result (-N (inl ssa<b))) +N (succ (succ a))) b _∣_ hcfDivB' (addMinus (inl ssa<b))
        go'' {succ (succ a)} {b} maxSum ssa<b ssa+b<maxsum indHyp | record { hcf = record { c = c ; c|a = c|a ; c|b = c|b ; hcf = hcf } ; extended1 = extended1 ; extended2 = extended2 ; extendedProof = inr extendedProof } = record { hcf = record { c = c ; c|a = c|b ; c|b = hcfDivB'' ; hcf = λ div prDivSSA prDivB → hcf div (dividesBothImpliesDividesDifference prDivB prDivSSA ssa<b) prDivSSA } ; extended2 = extended1; extended1 = extended2 +N extended1 ; extendedProof = inl (euclidLemma5 (succ a) b extended1 extended2 c ssa<b extendedProof) }
          where
            hcfDivB : c ∣ ((succ (succ a)) +N (subtractionNResult.result (-N (inl ssa<b))))
            hcfDivB = dividesBothImpliesDividesSum {c} {succ (succ a)} { subtractionNResult.result (-N (inl ssa<b))} c|b c|a
            hcfDivB' : c ∣ ((subtractionNResult.result (-N (inl ssa<b))) +N (succ (succ a)))
            hcfDivB' = identityOfIndiscernablesRight c ((succ (succ a)) +N (subtractionNResult.result (-N (inl ssa<b)))) ((subtractionNResult.result (-N (inl ssa<b))) +N (succ (succ a))) _∣_ hcfDivB (additionNIsCommutative (succ (succ a)) ( subtractionNResult.result (-N (inl ssa<b))))
            hcfDivB'' : c ∣ b
            hcfDivB'' = identityOfIndiscernablesRight c ((subtractionNResult.result (-N (inl ssa<b))) +N (succ (succ a))) b _∣_ hcfDivB' (addMinus (inl ssa<b))
        go' : (maxSum a b : ℕ) → (a +N b <N maxSum) → (∀ y → y <N maxSum → P y) → extendedHcf a b
        go' maxSum a b a+b<maxsum indHyp with orderIsTotal a b
        go' maxSum a b a+b<maxsum indHyp | inl (inl a<b) = go'' maxSum a<b a+b<maxsum indHyp
        go' maxSum a b a+b<maxsum indHyp | inl (inr b<a) = reverseHCF (go'' maxSum b<a (identityOfIndiscernablesLeft (a +N b) maxSum (b +N a) _<N_ a+b<maxsum (additionNIsCommutative a b)) indHyp)
        go' maxSum a .a _ indHyp | inr refl = record { hcf = record { c = a ; c|a = aDivA a ; c|b = aDivA a ; hcf = λ _ _ z → z } ; extended1 = 0 ; extended2 = 1 ; extendedProof = inr s}
          where
            s : a *N zero +N a ≡ a *N 1
            s rewrite multiplicationNIsCommutative a zero | productWithOneRight a = refl
        go : ∀ x → (∀ y → y <N x → P y) → P x
        go maxSum indHyp = λ a b a+b<maxSum → go' maxSum a b a+b<maxSum indHyp
        inducted : ∀ x → P x
        inducted = rec <NWellfounded P go

    divisorIsSmaller : {a b : ℕ} → a ∣ succ b → succ b <N a → False
    divisorIsSmaller {a} {b} (divides record { quot = zero ; rem = .0 ; pr = pr } refl) sb<a rewrite addZeroRight (a *N zero) = go
      where
        go : False
        go rewrite productZeroIsZeroRight a = naughtE pr
    divisorIsSmaller {a} {b} (divides record { quot = (succ quot) ; rem = .0 ; pr = pr } refl) sb<a rewrite addZeroRight (a *N succ quot) = go
      where
        go : False
        go rewrite equalityCommutative pr = go'
          where
            go' : False
            go' rewrite multiplicationNIsCommutative a (succ quot) = cannotAddAndEnlarge' sb<a

    primeDivisorIs1OrP : {a p : ℕ} → (prime : Prime p) → (a ∣ p) → (a ≡ 1) || (a ≡ p)
    primeDivisorIs1OrP {zero} {zero} prime a|p = inr refl
    primeDivisorIs1OrP {zero} {succ p} prime a|p = exFalso (zeroDividesNothing p a|p)
    primeDivisorIs1OrP {succ zero} {p} prime a|p = inl refl
    primeDivisorIs1OrP {succ (succ a)} {p} prime a|p with orderIsTotal (succ (succ a)) p
    primeDivisorIs1OrP {succ (succ a)} {p} prime a|p | inl (inl ssa<p) = go p prime a|p ssa<p
      where
        go : (n : ℕ) → Prime n → succ (succ a) ∣ n → succ (succ a) <N n → (succ (succ a) ≡ 1) || (succ (succ a) ≡ p)
        go zero pr x|n n<n = exFalso (zeroIsNotPrime pr)
        go (succ zero) pr x|n n<n = exFalso (oneIsNotPrime pr)
        go (succ (succ n)) pr x|n n<n = inl ((Prime.pr pr) {succ (succ a)} x|n n<n (succIsPositive (succ a)))
    primeDivisorIs1OrP {succ (succ a)} {zero} prime a|p | inl (inr x) = exFalso (zeroIsNotPrime prime)
    primeDivisorIs1OrP {succ (succ a)} {succ p} prime a|p | inl (inr x) = exFalso (divisorIsSmaller {succ (succ a)} {p} a|p x)
    primeDivisorIs1OrP {succ (succ a)} {p} prime a|p | inr x = inr x

    hcfPrimeIsOne' : {p : ℕ} → {a : ℕ} → (Prime p) → (0 <N divisionAlgResult.rem (divisionAlg p a)) → (extendedHcf.c (euclid a p) ≡ 1) || (extendedHcf.c (euclid a p) ≡ p)
    hcfPrimeIsOne' {p} {a} pPrime pCoprimeA with euclid a p
    hcfPrimeIsOne' {p} {a} pPrime pCoprimeA | record { hcf = record { c = hcf ; c|a = hcf|a ; c|b = hcf|p ; hcf = hcfPr } } with divisionAlg p a
    hcfPrimeIsOne' {p} {a} pPrime pCoprimeA | record { hcf = record { c = hcf ; c|a = hcf|a ; c|b = hcf|p ; hcf = hcfPr } } | record { quot = quot ; rem = rem ; pr = prPDivA } with primeDivisorIs1OrP pPrime hcf|p
    hcfPrimeIsOne' {p} {a} pPrime pCoprimeA | record { hcf = record { c = hcf ; c|a = hcf|a ; c|b = hcf|p ; hcf = hcfPr } } | record { quot = quot ; rem = rem ; pr = prPDivA } | inl x = inl x
    hcfPrimeIsOne' {p} {a} pPrime pCoprimeA | record { hcf = record { c = hcf ; c|a = hcf|a ; c|b = hcf|p ; hcf = hcfPr } } | record { quot = quot ; rem = rem ; pr = prPDivA } | inr x = inr x

    divisionDecidable : (a b : ℕ) → (a ∣ b) || ((a ∣ b) → False)
    divisionDecidable zero zero = inl (aDivA zero)
    divisionDecidable zero (succ b) = inr f
      where
        f : zero ∣ succ b → False
        f (divides record { quot = quot ; rem = rem ; pr = pr ; remIsSmall = remIsSmall } x) rewrite x = naughtE pr
    divisionDecidable (succ a) b with divisionAlg (succ a) b
    divisionDecidable (succ a) b | record { quot = quot ; rem = zero ; pr = pr ; remIsSmall = remSmall } = inl (divides (record { quot = quot ; rem = zero ; pr = pr ; remIsSmall = remSmall }) refl)
    divisionDecidable (succ a) b | record { quot = b/a ; rem = succ rem ; pr = prANotDivB ; remIsSmall = inr p } = naughtE (equalityCommutative p)
    divisionDecidable (succ a) b | record { quot = b/a ; rem = succ rem ; pr = prANotDivB ; remIsSmall = inl p } = inr f
      where
        f : (succ a) ∣ b → False
        f (divides record { quot = b/a' ; rem = .0 ; pr = pr } refl) rewrite addZeroRight ((succ a) *N b/a') = naughtE (modUniqueLemma {zero} {succ rem} {succ a} b/a' b/a (succIsPositive a) p comp')
          where
            comp : (succ a) *N b/a' ≡ (succ a) *N b/a +N succ rem
            comp = transitivity pr (equalityCommutative prANotDivB)
            comp' : (succ a) *N b/a' +N zero ≡ (succ a) *N b/a +N succ rem
            comp' rewrite addZeroRight (succ a *N b/a') = comp

    doesNotDivideImpliesNonzeroRem : (a b : ℕ) → ((a ∣ b) → False) → 0 <N divisionAlgResult.rem (divisionAlg a b)
    doesNotDivideImpliesNonzeroRem a b pr with divisionAlg a b
    doesNotDivideImpliesNonzeroRem a b pr | record { quot = quot ; rem = rem ; pr = divAlgPr ; remIsSmall = remIsSmall } with zeroIsValidRem rem
    doesNotDivideImpliesNonzeroRem a b pr | record { quot = quot ; rem = rem ; pr = divAlgPr ; remIsSmall = remIsSmall } | inl x = x
    doesNotDivideImpliesNonzeroRem a b pr | record { quot = quot ; rem = rem ; pr = divAlgPr ; remIsSmall = remIsSmall } | inr x = exFalso (pr aDivB)
      where
        aDivB : a ∣ b
        aDivB = divides (record { quot = quot ; rem = rem ; pr = divAlgPr ; remIsSmall = remIsSmall }) x

    hcfPrimeIsOne : {p : ℕ} → {a : ℕ} → (Prime p) → ((p ∣ a) → False) → extendedHcf.c (euclid a p) ≡ 1
    hcfPrimeIsOne {p} {a} pPrime pr with hcfPrimeIsOne' {p} {a} pPrime (doesNotDivideImpliesNonzeroRem p a pr)
    hcfPrimeIsOne {p} {a} pPrime pr | inl x = x
    hcfPrimeIsOne {p} {a} pPrime pr | inr x with euclid a p
    hcfPrimeIsOne {p} {a} pPrime pr | inr x | record { hcf = record { c = c ; c|a = c|a ; c|b = c|b ; hcf = hcf } ; extended1 = extended1 ; extended2 = extended2 ; extendedProof = extendedProof } rewrite x = exFalso (pr c|a)

    reduceEquationMod : {a b c : ℕ} → (d : ℕ) → (a ∣ b) → (a ∣ c) → b ≡ c +N d → a ∣ d
    reduceEquationMod {a} {b} {c} 0 a|b a|c pr = aDivZero a
    reduceEquationMod {a} {b} {c} (succ d) (divides record { quot = b/a ; rem = .0 ; pr = prb/a ; remIsSmall = r1 } refl) (divides record { quot = c/a ; rem = .0 ; pr = prc/a ; remIsSmall = r2 } refl) b=c+d = identityOfIndiscernablesRight a (subtractionNResult.result (-N (inl c<b))) (succ d) _∣_ a|b-c ex'
      where
        c<b : c <N b
        c<b rewrite succExtracts c d | additionNIsCommutative c d = le d (equalityCommutative b=c+d)
        a|b-c : a ∣ subtractionNResult.result (-N (inl c<b))
        a|b-c = dividesBothImpliesDividesDifference (divides record { quot = b/a ; rem = 0 ; pr = prb/a ; remIsSmall = r1} refl) (divides record { quot = c/a ; rem = 0 ; pr = prc/a ; remIsSmall = r2} refl) c<b
        ex : subtractionNResult.result (-N {c} {b} (inl c<b)) ≡ subtractionNResult.result (-N {0} {succ d} (inl (succIsPositive d)))
        ex = equivalentSubtraction c (succ d) b 0 (c<b) (succIsPositive d) (equalityCommutative (identityOfIndiscernablesLeft b (c +N succ d) (b +N 0) _≡_ b=c+d (equalityCommutative (addZeroRight b))))
        ex' : subtractionNResult.result (-N {c} {b} (inl c<b)) ≡ succ d
        ex' = identityOfIndiscernablesRight (subtractionNResult.result (-N (inl c<b))) (subtractionNResult.result (-N (inl (succIsPositive d)))) (succ d) _≡_ ex refl

    primesArePrime : {p : ℕ} → {a b : ℕ} → (Prime p) → p ∣ (a *N b) → (p ∣ a) || (p ∣ b)
    primesArePrime {p} {a} {b} pPrime pr with divisionDecidable p a
    primesArePrime {p} {a} {b} pPrime pr | inl p|a = inl p|a
    primesArePrime {p} {a} {b} pPrime (divides record {quot = ab/p ; rem = .0 ; pr = p|ab ; remIsSmall = _ } refl) | inr notp|a = inr (answer ex'')
      where
        euc : extendedHcf a p
        euc = euclid a p
        h : extendedHcf.c euc ≡ 1
        h = hcfPrimeIsOne {p} {a} pPrime notp|a
        x = extendedHcf.extended1 euc
        y = extendedHcf.extended2 euc
        extended : ((a *N x) ≡ p *N y +N extendedHcf.c euc) || (a *N x +N extendedHcf.c euc ≡ p *N y)
        extended = extendedHcf.extendedProof euc
        extended' : (a *N x ≡ p *N y +N 1) || (a *N x +N 1 ≡ p *N y)
        extended' rewrite equalityCommutative h = extended
        extended'' : ((a *N x ≡ p *N y +N 1) || (a *N x +N 1 ≡ p *N y)) → (b *N (a *N x) ≡ b *N (p *N y +N 1)) || (b *N (a *N x +N 1) ≡ b *N (p *N y))
        extended'' (inl z) = inl (applyEquality (λ t → b *N t) z)
        extended'' (inr z) = inr (applyEquality (λ t → b *N t) z)
        ex : (b *N (a *N x) ≡ b *N (p *N y +N 1)) || (b *N (a *N x +N 1) ≡ b *N (p *N y)) → ((b *N a) *N x ≡ (b *N (p *N y) +N b)) || (((b *N a) *N x +N b) ≡ b *N (p *N y))
        ex (inl z) rewrite multiplicationNIsAssociative b a x | productDistributes b (p *N y) 1 | productWithOneRight b = inl z
        ex (inr z) rewrite productDistributes b (a *N x) 1 | multiplicationNIsAssociative b a x | productWithOneRight b = inr z
        ex' : ((a *N b) *N x ≡ ((p *N y) *N b +N b)) || (((a *N b) *N x +N b) ≡ (p *N y) *N b)
        ex' rewrite multiplicationNIsCommutative a b | multiplicationNIsCommutative (p *N y) b = ex (extended'' extended')
        ex'' : ((a *N b) *N x ≡ (p *N (y *N b) +N b)) || (((a *N b) *N x +N b) ≡ p *N (y *N b))
        ex'' rewrite multiplicationNIsAssociative (p) y b = ex'
        inter1 : p ∣ (a *N b) *N x
        inter1 = divides (record {quot = ab/p *N x ; rem = 0 ; pr = g ; remIsSmall = zeroIsValidRem p }) refl
          where
            g' : p *N ab/p ≡ a *N b
            g' rewrite addZeroRight (p *N ab/p) = p|ab
            g'' : p *N (ab/p *N x) ≡ (a *N b) *N x
            g'' rewrite multiplicationNIsAssociative (p) ab/p x = applyEquality (λ t → t *N x) g'
            g : p *N (ab/p *N x) +N 0 ≡ (a *N b) *N x
            g rewrite addZeroRight (p *N (ab/p *N x)) = g''
        inter2 : p ∣ (p *N (y *N b))
        inter2 = divides (record {quot = y *N b ; rem = 0 ; pr = addZeroRight (p *N (y *N b)) ; remIsSmall = zeroIsValidRem (p)}) refl
        answer : ((a *N b) *N x ≡ (p *N (y *N b) +N b)) || (((a *N b) *N x +N b) ≡ p *N (y *N b)) → (p ∣ b)
        answer (inl z) = reduceEquationMod {p} b inter1 inter2 z
        answer (inr z) = reduceEquationMod {p} b inter2 inter1 (equalityCommutative z)

    primesAreBiggerThanOne : {p : ℕ} → Prime p → (1 <N p)
    primesAreBiggerThanOne {zero} record { p>1 = (le x ()) ; pr = pr }
    primesAreBiggerThanOne {succ zero} pr = exFalso (oneIsNotPrime pr)
    primesAreBiggerThanOne {succ (succ p)} pr = succPreservesInequality (succIsPositive p)

    primesAreBiggerThanZero : {p : ℕ} → Prime p → 0 <N p
    primesAreBiggerThanZero {p} pr = orderIsTransitive (succIsPositive 0) (primesAreBiggerThanOne pr)

    record notDividedByLessThan (a : ℕ) (firstPossibleDivisor : ℕ) : Set where
        field
            previousDoNotDivide : ∀ x → 1 <N x → x <N firstPossibleDivisor → x ∣ a → False

    alternativePrime : {a : ℕ} → 1 <N a → notDividedByLessThan a a → Prime a
    alternativePrime {a} 1<a record { previousDoNotDivide = previousDoNotDivide } = record { pr = pr ; p>1 = 1<a}
      where
        pr : {x : ℕ} → (x|a : x ∣ a) (x<a : x <N a) (0<x : zero <N x) → x ≡ 1
        pr {zero} _ _ (le x ())
        pr {succ zero} _ _ _ = refl
        pr {succ (succ x)} x|a x<a 0<x = exFalso (previousDoNotDivide (succ (succ x)) (succPreservesInequality (succIsPositive x)) x<a x|a)

    divisibilityTransitive : {a b c : ℕ} → a ∣ b → b ∣ c → a ∣ c
    divisibilityTransitive {a} {b} {c} (divides record { quot = b/a ; rem = .0 ; pr = prDivAB ; remIsSmall = remIsSmallAB } refl) (divides record { quot = c/b ; rem = .0 ; pr = prDivBC ; remIsSmall = remIsSmallBC } refl) = divides record { quot = b/a *N c/b ; rem = 0 ; pr = p ; remIsSmall = zeroIsValidRem a } refl
      where
        p : a *N (b/a *N c/b) +N 0 ≡ c
        p rewrite addZeroRight (a *N (b/a *N c/b)) | addZeroRight (b *N c/b) | addZeroRight (a *N b/a) | multiplicationNIsAssociative a b/a c/b | prDivAB | prDivBC = refl

    compositeOrPrimeLemma : {a b : ℕ} → notDividedByLessThan b a → a ∣ b → {i : ℕ} → (i ∣ a) → (i <N a) → (0 <N i) → i ≡ 1
    compositeOrPrimeLemma {a} {b} record { previousDoNotDivide = previousDoNotDivide } a|b {zero} i|a i<a 0<i = exFalso (lessIrreflexive 0<i)
    compositeOrPrimeLemma {a} {b} record { previousDoNotDivide = previousDoNotDivide } a|b {succ zero} i|a i<a 0<i = refl
    compositeOrPrimeLemma {a} {b} record { previousDoNotDivide = previousDoNotDivide } a|b {succ (succ i)} i|a i<a 0<i = exFalso (previousDoNotDivide (succ (succ i)) (succPreservesInequality (succIsPositive i)) i<a (divisibilityTransitive i|a a|b) )

    compositeOrPrime : (a : ℕ) → (1 <N a) → (Composite a) || (Prime a)
    compositeOrPrime a pr = go''' go''
      where
        base : notDividedByLessThan a 2
        base = record { previousDoNotDivide = λ x 1<x x<2 _ → noIntegersBetweenXAndSuccX 1 1<x x<2 }
        go : {firstPoss : ℕ} → notDividedByLessThan a firstPoss → ((notDividedByLessThan a (succ firstPoss)) || ((firstPoss ∣ a) && (firstPoss <N a))) || (firstPoss ≡ a)
        go' : (firstPoss : ℕ) → (((notDividedByLessThan a firstPoss) || (Composite a))) || (notDividedByLessThan a a)
        go'' : (notDividedByLessThan a a) || (Composite a)
        go'' with go' a
        ... | inr x = inl x
        ... | inl x = x
        go''' : ((notDividedByLessThan a a) || (Composite a)) → ((Composite a) || (Prime a))
        go''' (inl x) = inr (alternativePrime pr x)
        go''' (inr x) = inl x
        go' (zero) = inl (inl (record { previousDoNotDivide = λ x 1<x x<0 _ → zeroNeverGreater x<0 }))
        go' (succ 0) = inl (inl (record { previousDoNotDivide = λ x 1<x x<1 _ → orderIsIrreflexive x<1 1<x }))
        go' (succ (succ zero)) = inl (inl base)
        go' (succ (succ (succ firstPoss))) with go' (succ (succ firstPoss))
        go' (succ (succ (succ firstPoss))) | inl (inl x) with go {succ (succ firstPoss)} x
        go' (succ (succ (succ firstPoss))) | inl (inl x) | inl (inl x1) = inl (inl x1)
        go' (succ (succ (succ firstPoss))) | inl (inl x) | inl (inr x1) = inl (inr record { noSmallerDivisors = λ i i<ssFP 1<i i|a → notDividedByLessThan.previousDoNotDivide x i 1<i i<ssFP i|a ; n>1 = pr ; divisor = succ (succ firstPoss) ; dividesN = _&&_.fst x1 ; divisorLessN = _&&_.snd x1 ; divisorNot1 = succPreservesInequality (succIsPositive firstPoss) ; divisorPrime = record { p>1 = succPreservesInequality (succIsPositive firstPoss) ; pr = compositeOrPrimeLemma {succ (succ firstPoss)} {a} x (_&&_.fst x1) } })
        go' (succ (succ (succ firstPoss))) | inl (inl x) | inr y rewrite y = inr x
        go' (succ (succ (succ firstPoss))) | inl (inr x) = inl (inr x)
        go' (succ (succ (succ firstPoss))) | inr x = inr x

        go {zero} pr = inl (inl (record { previousDoNotDivide = λ x 1<x x<1 _ → orderIsIrreflexive x<1 1<x}))
        go {succ firstPoss} knownCoprime with orderIsTotal (succ firstPoss) a
        go {succ firstPoss} knownCoprime | inr x = inr x
        go {succ firstPoss} knownCoprime | inl (inl sFP<a) with divisionAlg (succ firstPoss) a
        go {succ firstPoss} knownCoprime | inl (inl sFP<a) | record { quot = quot ; rem = zero ; pr = pr ; remIsSmall = remIsSmall } = inl (inr record { fst = (divides (record { quot = quot ; rem = zero ; remIsSmall = remIsSmall ; pr = pr}) refl) ; snd = sFP<a })
        go {succ firstPoss} knownCoprime | inl (inl sFP<a) | record { quot = quot ; rem = succ rem ; pr = pr ; remIsSmall = remIsSmall } = inl next
          where
            previous : ∀ x → 1 <N x → x <N succ firstPoss → x ∣ a → False
            previous = notDividedByLessThan.previousDoNotDivide knownCoprime
            next : notDividedByLessThan a (succ (succ firstPoss)) || (((succ firstPoss) ∣ a) && (succ firstPoss <N a))
            next with divisionAlg (succ firstPoss) a
            next | record { quot = quot ; rem = zero ; pr = pr ; remIsSmall = remIsSmall } = inr (record { fst = divides record { quot = quot ; rem = zero ; pr = pr ; remIsSmall = remIsSmall } refl ; snd = sFP<a } )
            next | record { quot = quot ; rem = succ rem ; pr = pr ; remIsSmall = remIsSmall } = inl record { previousDoNotDivide = (next' record { quot = quot ; rem = succ rem ; pr = pr ; remIsSmall = remIsSmall } (succIsPositive rem)) }
              where
                next' : (res : divisionAlgResult (succ firstPoss) a) → (pr : 0 <N divisionAlgResult.rem res) → (x : ℕ) → 1 <N x → x <N succ (succ firstPoss) → x ∣ a → False
                next' (res) (prDiv) x 1<x x<ssFirstposs x|a with orderIsTotal x (succ firstPoss)
                next' (res) (prDiv) x 1<x x<ssFirstposs x|a | inl (inl x<sFirstPoss) = previous x 1<x x<sFirstPoss x|a
                next' (res) (prDiv) x 1<x x<ssFirstposs x|a | inl (inr sFirstPoss<x) = noIntegersBetweenXAndSuccX (succ firstPoss) sFirstPoss<x x<ssFirstposs
                next' res prDiv x 1<x x<ssFirstposs (divides res1 x1) | inr x=sFirstPoss rewrite equalityCommutative x=sFirstPoss = g
                  where
                    g : False
                    g with modIsUnique res res1
                    ... | r rewrite r = lessImpliesNotEqual prDiv (equalityCommutative x1)
        go {succ firstPoss} record { previousDoNotDivide = previousDoNotDivide } | inl (inr a<sFP) = exFalso (previousDoNotDivide a pr a<sFP (aDivA a))

    primeDivPrimeImpliesEqual : {p1 p2 : ℕ} → Prime p1 → Prime p2 → p1 ∣ p2 → p1 ≡ p2
    primeDivPrimeImpliesEqual {p1} {p2} pr1 pr2 p1|p2 with orderIsTotal p1 p2
    primeDivPrimeImpliesEqual {p1} {p2} pr1 record { p>1 = p>1 ; pr = pr } p1|p2 | inl (inl p1<p2) with pr p1|p2 p1<p2 (primesAreBiggerThanZero {p1} pr1)
    ... | p1=1 = exFalso (oneIsNotPrime contr)
      where
        contr : Prime 1
        contr rewrite p1=1 = pr1
    primeDivPrimeImpliesEqual {p1} {zero} pr1 pr2 p1|p2 | inl (inr p1>p2) = exFalso (zeroIsNotPrime pr2)
    primeDivPrimeImpliesEqual {p1} {succ p2} pr1 pr2 p1|p2 | inl (inr p1>p2) = exFalso (divisorIsSmaller p1|p2 p1>p2)
    primeDivPrimeImpliesEqual {p1} {p2} pr1 pr2 p1|p2 | inr p1=p2 = p1=p2

    mult1Lemma : {a b : ℕ} → a *N succ b ≡ 1 → (a ≡ 1) && (b ≡ 0)
    mult1Lemma {a} {b} pr = record { fst = _&&_.fst p ; snd = q}
      where
        p : (a ≡ 1) && (succ b ≡ 1)
        p = productOneImpliesOperandsOne pr
        q : b ≡ zero
        q = succInjective (_&&_.snd p)

    oneHasNoDivisors : {a : ℕ} → a ∣ 1 → a ≡ 1
    oneHasNoDivisors {a} (divides record { quot = zero ; rem = .0 ; pr = pr ; remIsSmall = remIsSmall } refl) rewrite addZeroRight (a *N zero) | multiplicationNIsCommutative a zero | addZeroRight a = naughtE pr
    oneHasNoDivisors {a} (divides record { quot = (succ quot) ; rem = .0 ; pr = pr ; remIsSmall = remIsSmall } refl) rewrite addZeroRight (a *N succ quot) = _&&_.fst (mult1Lemma pr)

    notSmallerMeansGE : {a b : ℕ} → (a <N b → False) → b ≤N a
    notSmallerMeansGE {a} {b} notA<b with orderIsTotal a b
    notSmallerMeansGE {a} {b} notA<b | inl (inl x) = exFalso (notA<b x)
    notSmallerMeansGE {a} {b} notA<b | inl (inr x) = inl x
    notSmallerMeansGE {a} {b} notA<b | inr x = inr (equalityCommutative x)