module Monad.Instance.Delay.Strong {o ℓ e} (ambient : Ambient o ℓ e) (D : DelayM ambient) where
  open Ambient ambient
  open HomReasoning
  open Equiv
  open MP C
  open MR C
  open M C
  open F-Coalgebra-Morphism renaming (f to u)

  module _ where
    open DelayM D
    open Functor
    open RMonad kleisli using (extend; extend-≈) renaming (assoc to k-assoc; identityʳ to k-identityʳ)
    open Monad monad using (η; μ)

    -- TODO change 'coinduction' proofs, move the two proofs i.e. f ≈ ! and ! ≈ g to the where clause
    module τ-mod (P : Category.Obj (CProduct C C)) where
      private
        X = proj₁ P
        Y = proj₂ P
      open Terminal (coalgebras (X × Y))

      τ : X × D₀ Y ⇒ D₀ (X × Y)
      τ = u (! {A = record { A = X × D₀ Y ; α = distributeˡ⁻¹ ∘ (idC ⁂ out) }})

      τ-law : out ∘ τ ≈ (idC +₁ τ) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)
      τ-law = commutes (! {A = record { A = X × D₀ Y ; α = distributeˡ⁻¹ ∘ (idC ⁂ out) }})

      τ-helper : τ ∘ (idC ⁂ out⁻¹) ≈ out⁻¹ ∘ (idC +₁ τ) ∘ distributeˡ⁻¹
      τ-helper = begin 
        τ ∘ (idC ⁂ out⁻¹)                                                  ≈⟨ introˡ (_≅_.isoˡ out-≅) ⟩ 
        (out⁻¹ ∘ out) ∘ τ ∘ (idC ⁂ out⁻¹)                                  ≈⟨ pullʳ (pullˡ τ-law) ⟩
        out⁻¹ ∘ ((idC +₁ τ) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)) ∘ (idC ⁂ out⁻¹) ≈⟨ refl⟩∘⟨ (assoc ○ refl⟩∘⟨ assoc) ⟩
        out⁻¹ ∘ (idC +₁ τ) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) ∘ (idC ⁂ out⁻¹)   ≈⟨ refl⟩∘⟨ (refl⟩∘⟨ (elimʳ (⁂∘⁂ ○ (⁂-cong₂ identity² (_≅_.isoʳ out-≅)) ○ ((⟨⟩-cong₂ identityˡ identityˡ) ○ ⁂-η)))) ⟩
        out⁻¹ ∘ (idC +₁ τ) ∘ distributeˡ⁻¹                                 ∎

      τ-now : τ ∘ (idC ⁂ now) ≈ now
      τ-now = begin 
        τ ∘ (idC ⁂ now)                                   ≈⟨ refl⟩∘⟨ sym (⁂∘⁂ ○ (⁂-cong₂ identity² refl)) ⟩ 
        τ ∘ (idC ⁂ out⁻¹) ∘ (idC ⁂ i₁)                    ≈⟨ pullˡ τ-helper ⟩
        (out⁻¹ ∘ (idC +₁ τ) ∘ distributeˡ⁻¹) ∘ (idC ⁂ i₁) ≈⟨ pullʳ (pullʳ distributeˡ⁻¹-i₁) ⟩
        out⁻¹ ∘ (idC +₁ τ) ∘ i₁                           ≈⟨ refl⟩∘⟨ +₁∘i₁ ⟩
        out⁻¹ ∘ i₁ ∘ idC                                  ≈⟨ refl⟩∘⟨ identityʳ ⟩
        now                                               ∎
      
      ▷∘τ : τ ∘ (idC ⁂ ▷) ≈ ▷ ∘ τ
      ▷∘τ = begin 
        τ ∘ (idC ⁂ ▷)                                     ≈⟨ refl⟩∘⟨ (sym (⁂∘⁂ ○ ⁂-cong₂ identity² refl)) ⟩ 
        τ ∘ (idC ⁂ out⁻¹) ∘ (idC ⁂ i₂)                    ≈⟨ pullˡ τ-helper ⟩ 
        (out⁻¹ ∘ (idC +₁ τ) ∘ distributeˡ⁻¹) ∘ (idC ⁂ i₂) ≈⟨ pullʳ (pullʳ distributeˡ⁻¹-i₂) ⟩ 
        out⁻¹ ∘ (idC +₁ τ) ∘ i₂                           ≈⟨ refl⟩∘⟨ +₁∘i₂ ⟩ 
        out⁻¹ ∘ i₂ ∘ τ                                    ≈⟨ sym-assoc ⟩ 
        ▷ ∘ τ                                             ∎

      τ-unique : (t : X × D₀ Y ⇒ D₀ (X × Y)) → (out ∘ t ≈ (idC +₁ t) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)) → t ≈ τ
      τ-unique t t-commutes = sym (!-unique (record { f = t ; commutes = t-commutes }))      

    open τ-mod

    strength : Strength monoidal monad
    strength = record
      { strengthen = ntHelper (record 
        { η = τ
        ; commute = commute' })
      ; identityˡ = identityˡ' -- triangle
      ; η-comm = λ {A} {B} → τ-now (A , B)
      ; μ-η-comm = μ-η-comm' -- μ-τ
      ; strength-assoc = strength-assoc' -- square
      }
      where
        identityˡ' : ∀ {X : Obj} → extend (now ∘ π₂) ∘ τ (Terminal.⊤ terminal , X) ≈ π₂
        identityˡ' {X} = begin
          extend (now ∘ π₂) ∘ τ _       ≈⟨ sym (Terminal.!-unique (coalgebras X) {A = record { A = Terminal.⊤ terminal × D₀ X ; α = (π₂ +₁ idC) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) }} (record { f = extend (now ∘ π₂) ∘ τ _ ; commutes = begin 
            out ∘ extend (now ∘ π₂) ∘ τ _                                                                                                 ≈⟨ refl⟩∘⟨ (refl⟩∘⟨ (sym identityʳ)) ⟩ 
            out ∘ extend (now ∘ π₂) ∘ τ _ ∘ idC                                                                                           ≈⟨ pullˡ diag₃ ⟩ 
            ((π₂ +₁ extend (now ∘ π₂)) ∘ out) ∘ τ _ ∘ idC                                                                                 ≈⟨ pullʳ (pullˡ (diag₂ (Terminal.⊤ terminal , X))) ⟩ 
            (π₂ +₁ extend (now ∘ π₂)) ∘ ((idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)) ∘ idC                                                ≈⟨ refl⟩∘⟨ (pullʳ (assoc ○ sym diag₁)) ⟩ 
            (π₂ +₁ extend (now ∘ π₂)) ∘ (idC +₁ τ _) ∘ (⟨ Terminal.! terminal , idC ⟩ +₁ idC) ∘ (π₂ +₁ idC) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) ≈⟨ refl⟩∘⟨ refl⟩∘⟨ (pullˡ (+₁∘+₁ ○ +₁-cong₂ (_≅_.isoˡ ⊤×A≅A) identity² ○ CC.+-unique ((identity² ⟩∘⟨refl) ○ id-comm-sym) ((identity² ⟩∘⟨refl) ○ id-comm-sym)) ) ⟩ 
            (π₂ +₁ extend (now ∘ π₂)) ∘ (idC +₁ τ _) ∘ (idC ∘ idC) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)                                          ≈⟨ pullˡ +₁∘+₁ ⟩ 
            (π₂ ∘ idC +₁ extend (now ∘ π₂) ∘ τ _) ∘ (idC ∘ idC) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)                                             ≈⟨ (+₁-cong₂ id-comm refl) ⟩∘⟨refl ⟩ 
            (idC ∘ π₂ +₁ extend (now ∘ π₂) ∘ τ _) ∘ (idC ∘ idC) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)                                             ≈˘⟨ pullˡ +₁∘+₁ ⟩ 
            (idC +₁ extend (now ∘ π₂)) ∘ (π₂ +₁ τ _) ∘ (idC ∘ idC) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)                                          ≈⟨ refl⟩∘⟨ (refl⟩∘⟨ (elimˡ identity²)) ⟩ 
            (idC +₁ extend (now ∘ π₂)) ∘ (π₂ +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)                                                        ≈˘⟨ refl⟩∘⟨ ((+₁-cong₂ identityˡ identityʳ) ⟩∘⟨refl) ⟩ 
            (idC +₁ extend (now ∘ π₂)) ∘ (idC ∘ π₂ +₁ τ _ ∘ idC) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)                                            ≈˘⟨ pullʳ (pullˡ +₁∘+₁) ⟩ 
            ((idC +₁ extend (now ∘ π₂)) ∘ (idC +₁ τ _)) ∘ (π₂ +₁ idC) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)                                       ≈˘⟨ (homomorphism (X +-)) ⟩∘⟨refl ⟩ 
            (idC +₁ extend (now ∘ π₂) ∘ τ _) ∘ (π₂ +₁ idC) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)                                                  ∎ })) ⟩
          u (Terminal.! (coalgebras X)) ≈⟨ Terminal.!-unique (coalgebras X) {A = record { A = Terminal.⊤ terminal × D₀ X ; α = (π₂ +₁ idC) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) }} (record { f = π₂ ; commutes = begin 
            out ∘ π₂                                                ≈˘⟨ π₂∘⁂ ⟩ 
            π₂ ∘ (idC ⁂ out)                                        ≈˘⟨ pullˡ distributeˡ⁻¹-π₂ ⟩
            (π₂ +₁ π₂) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)                ≈˘⟨ (+₁-cong₂ identityˡ identityʳ) ⟩∘⟨refl ⟩ 
            (idC ∘ π₂ +₁ π₂ ∘ idC) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)    ≈˘⟨ pullˡ +₁∘+₁ ⟩ 
            (idC +₁ π₂) ∘ (π₂ +₁ idC) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) ∎ }) ⟩
          π₂                            ∎ 
          where
            diag₁ : (⟨ Terminal.! terminal , idC ⟩ +₁ idC) ∘ (π₂ +₁ idC) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) ≈ distributeˡ⁻¹ {Terminal.⊤ terminal} {X} {D₀ X} ∘ (idC ⁂ out) ∘ idC
            diag₁ = begin 
              (⟨ Terminal.! terminal , idC ⟩ +₁ idC) ∘ (π₂ +₁ idC) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) ≈⟨ pullˡ +₁∘+₁ ⟩ 
              (⟨ Terminal.! terminal , idC ⟩ ∘ π₂ +₁ idC ∘ idC) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)    ≈⟨ +₁-cong₂ (_≅_.isoˡ ⊤×A≅A) identity² ⟩∘⟨ (refl⟩∘⟨ sym identityʳ) ⟩ 
              (idC +₁ idC) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) ∘ idC                                   ≈⟨ elimˡ (CC.+-unique id-comm-sym id-comm-sym) ⟩ 
              distributeˡ⁻¹ ∘ (idC ⁂ out) ∘ idC                                                  ∎
            diag₂ = τ-law
            diag₃ : out {X} ∘ extend (now ∘ π₂ {A = Terminal.⊤ terminal} {B = X}) ≈ (π₂ +₁ extend (now ∘ π₂)) ∘ out
            diag₃ = out-law π₂

        μ-η-comm' : ∀ {X Y} → extend idC ∘ (extend (now ∘ τ _)) ∘ τ _ ≈ τ (X , Y) ∘ (idC ⁂ extend idC)
        μ-η-comm' {X} {Y} = begin 
          extend idC ∘ (extend (now ∘ τ _)) ∘ τ _                                                                                                                                ≈⟨ sym (Terminal.!-unique (coalgebras (X × Y)) (record { f = extend idC ∘ (extend (now ∘ τ _)) ∘ τ _ ; commutes = begin 
            out ∘ extend idC ∘ extend (now ∘ τ (X , Y)) ∘ τ _                                                                                                  ≈⟨ refl⟩∘⟨ (pullˡ id*∘Dτ) ⟩
            out ∘ extend (τ (X , Y)) ∘ τ _                                                                                                                     ≈⟨ square ⟩
            [ out ∘ τ _ , i₂ ∘ extend (τ _) ] ∘ (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)                                                                     ≈⟨ sym-assoc ○ sym-assoc ○ (assoc ○ tri) ⟩∘⟨refl ⟩
            ((idC +₁ extend (τ _) ∘ τ _) ∘ [ (idC +₁ (idC ⁂ now)) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , i₂ ] ∘ distributeˡ⁻¹) ∘ (idC ⁂ out)                          ≈⟨ assoc ○ refl⟩∘⟨ assoc ⟩
            (idC +₁ (extend (τ _) ∘ τ _)) ∘ [ (idC +₁ (idC ⁂ now)) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , i₂ ] ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)                          ≈⟨ (+₁-cong₂ refl (sym (pullˡ id*∘Dτ))) ⟩∘⟨refl ⟩
            (idC +₁ (extend idC ∘ extend (now ∘ τ (X , Y)) ∘ τ _)) ∘ [ (idC +₁ (idC ⁂ now)) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , i₂ ] ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) ∎ })) ⟩
          u (Terminal.! (coalgebras (X × Y)) {A = record { A = X × D₀ (D₀ Y) ; α = [ (idC +₁ (idC ⁂ now)) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , i₂ ] ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) }}) ≈⟨ Terminal.!-unique (coalgebras (X × Y)) (record { f = τ _ ∘ (idC ⁂ extend idC) ; commutes = begin 
            out ∘ τ _ ∘ (idC ⁂ extend idC) ≈⟨ pullˡ (τ-law (X , Y)) ⟩ 
            ((idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)) ∘ (idC ⁂ extend idC)                                                               ≈⟨ pullʳ (pullʳ (⁂∘⁂ ○ ⁂-cong₂ identity² refl)) ⟩
            (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out ∘ extend idC)                                                                         ≈⟨ refl⟩∘⟨ refl⟩∘⟨ ⁂-cong₂ refl (extendlaw idC) ⟩
            (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ [ out ∘ idC , i₂ ∘ extend idC ] ∘ out)                                                    ≈⟨ refl⟩∘⟨ refl⟩∘⟨ ⁂-cong₂ refl (([]-cong₂ identityʳ refl) ⟩∘⟨refl) ⟩
            (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ [ out , i₂ ∘ extend idC ] ∘ out)                                                          ≈⟨ refl⟩∘⟨ refl⟩∘⟨ sym (⁂∘⁂ ○ ⁂-cong₂ identity² refl) ⟩
            (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ [ out , i₂ ∘ extend idC ]) ∘ (idC ⁂ out)                                                  ≈⟨ sym-assoc ○ pullˡ (assoc ○ tri₂) ⟩
            ((idC +₁ τ _ ∘ (idC ⁂ extend idC)) ∘ [ (idC +₁ (idC ⁂ now)) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , i₂ ] ∘ distributeˡ⁻¹) ∘ (idC ⁂ out) ≈⟨ assoc ○ refl⟩∘⟨ assoc ⟩
            (idC +₁ τ _ ∘ (idC ⁂ extend idC)) ∘ [ (idC +₁ (idC ⁂ now)) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , i₂ ] ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)   ∎ }) ⟩ 
          τ _ ∘ (idC ⁂ extend idC)                                                                                                                                               ∎
          where
            -- diagram: https://q.uiver.app/#q=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
            square : out ∘ extend (τ (X , Y)) ∘ τ _ ≈ [ out ∘ τ _ , i₂ ∘ extend (τ _) ] ∘ (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)
            square = begin 
              out ∘ extend (τ (X , Y)) ∘ τ _                                                 ≈⟨ pullˡ (extendlaw (τ (X , Y))) ⟩ 
              ([ out ∘ τ (X , Y) , i₂ ∘ extend (τ (X , Y)) ] ∘ out) ∘ τ _                    ≈⟨ pullʳ (τ-law _) ⟩ 
              [ out ∘ τ _ , i₂ ∘ extend (τ _) ] ∘ (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) ∎
            tri : [ out ∘ τ _ , i₂ ∘ extend (τ (X , Y)) ] ∘ (idC +₁ τ _) ∘ distributeˡ⁻¹ ≈ (idC +₁ extend (τ _) ∘ τ _) ∘ [ (idC +₁ (idC ⁂ now)) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , i₂ ] ∘ distributeˡ⁻¹
            tri = begin 
              [ out ∘ τ _ , i₂ ∘ extend (τ (X , Y)) ] ∘ (idC +₁ τ _) ∘ distributeˡ⁻¹                                                                  ≈⟨ pullˡ []∘+₁ ⟩ 
              [ (out ∘ τ _) ∘ idC , (i₂ ∘ extend (τ (X , Y))) ∘ τ _ ] ∘ distributeˡ⁻¹                                                                 ≈⟨ ([]-cong₂ (identityʳ ○ τ-law (X , Y)) assoc) ⟩∘⟨refl ⟩ 
              [ (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , i₂ ∘ extend (τ (X , Y)) ∘ τ _ ] ∘ distributeˡ⁻¹                                          ≈⟨ sym (([]-cong₂ ((+₁-cong₂ refl k-identityʳ) ⟩∘⟨refl) refl) ⟩∘⟨refl) ⟩ 
              [ (idC +₁ extend (τ _) ∘ now) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , i₂ ∘ extend (τ _) ∘ τ _ ] ∘ distributeˡ⁻¹                                 ≈⟨ sym (([]-cong₂ ((+₁-cong₂ identity² (pullʳ (τ-now (X , D₀ Y)))) ⟩∘⟨refl) refl) ⟩∘⟨refl) ⟩ 
              [ (idC ∘ idC +₁ (extend (τ _) ∘ τ _) ∘ (idC ⁂ now)) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , i₂ ∘ extend (τ _) ∘ τ _ ] ∘ distributeˡ⁻¹           ≈⟨ sym (([]-cong₂ (pullˡ +₁∘+₁) +₁∘i₂) ⟩∘⟨refl) ⟩ 
              [ (idC +₁ extend (τ _) ∘ τ _) ∘ (idC +₁ (idC ⁂ now)) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , (idC +₁ extend (τ _) ∘ τ _) ∘ i₂ ] ∘ distributeˡ⁻¹ ≈⟨ sym (pullˡ ∘[]) ⟩ 
              (idC +₁ extend (τ _) ∘ τ _) ∘ [ (idC +₁ (idC ⁂ now)) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , i₂ ] ∘ distributeˡ⁻¹                               ∎
            id*∘Dτ : extend idC ∘ extend (now ∘ τ _) ≈ extend (τ _)
            id*∘Dτ = begin 
              extend idC ∘ extend (now ∘ τ _) ≈⟨ sym k-assoc ⟩ 
              extend (extend idC ∘ now ∘ τ _) ≈⟨ extend-≈ (pullˡ k-identityʳ) ⟩
              extend (idC ∘ τ _)              ≈⟨ extend-≈ identityˡ ⟩ 
              extend (τ _)                    ∎
            tri₂' : (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ [ out , i₂ ∘ extend idC ]) ∘ [ (idC ⁂ i₁) , (idC ⁂ i₂) ] ≈ (idC +₁ τ _ ∘ (idC ⁂ extend idC)) ∘ [ (idC +₁ (idC ⁂ now)) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , i₂ ] ∘ distributeˡ⁻¹ ∘ distributeˡ
            tri₂' = begin 
              (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ [ out , i₂ ∘ extend idC ]) ∘ [ (idC ⁂ i₁) , (idC ⁂ i₂) ]                                      ≈⟨ refl⟩∘⟨ (refl⟩∘⟨ ∘[]) ⟩ 
              (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ [ (idC ⁂ [ out , i₂ ∘ extend idC ]) ∘ (idC ⁂ i₁) , (idC ⁂ [ out , i₂ ∘ extend idC ]) ∘ (idC ⁂ i₂) ]  ≈⟨ refl⟩∘⟨ refl⟩∘⟨ []-cong₂ ⁂∘⁂ ⁂∘⁂ ⟩
              (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ [ (idC ∘ idC ⁂ [ out , i₂ ∘ extend idC ] ∘ i₁) , (idC ∘ idC ⁂ [ out , i₂ ∘ extend idC ] ∘ i₂) ]      ≈⟨ refl⟩∘⟨ refl⟩∘⟨ []-cong₂ (⁂-cong₂ identity² inject₁) (⁂-cong₂ identity² inject₂) ⟩
              (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ [ idC ⁂ out , idC ⁂ i₂ ∘ extend idC ]                                                                ≈⟨ refl⟩∘⟨ ∘[] ⟩
              (idC +₁ τ _) ∘ [ distributeˡ⁻¹ ∘ (idC ⁂ out) , distributeˡ⁻¹ ∘ (idC ⁂ i₂ ∘ extend idC) ]                                            ≈⟨ ∘[] ⟩
              [ (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ i₂ ∘ extend idC) ]                             ≈⟨ sym ([]-cong₂ refl (pullʳ (pullʳ (⁂∘⁂ ○ ⁂-cong₂ identity² refl)))) ⟩
              [ (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , ((idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ i₂)) ∘ (idC ⁂ extend idC) ]                   ≈⟨ []-cong₂ refl ((refl⟩∘⟨ distributeˡ⁻¹-i₂) ⟩∘⟨refl) ⟩
              [ (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , ((idC +₁ τ _) ∘ i₂) ∘ (idC ⁂ extend idC) ]                                           ≈⟨ []-cong₂ refl (pushˡ +₁∘i₂) ⟩
              [ (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , i₂ ∘ τ _ ∘ (idC ⁂ extend idC) ]                                                      ≈⟨ []-cong₂ ((+₁-cong₂ refl (introʳ (⟨⟩-unique id-comm (id-comm ○ (sym k-identityʳ) ⟩∘⟨refl)))) ⟩∘⟨refl) refl ⟩
              [ (idC +₁ τ _ ∘ (idC ⁂ extend idC ∘ now)) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , i₂ ∘ τ _ ∘ (idC ⁂ extend idC) ]                           ≈⟨ []-cong₂ ((sym (+₁-cong₂ refl (refl⟩∘⟨ (⁂∘⁂ ○ ⁂-cong₂ identity² refl)))) ⟩∘⟨refl) refl ⟩
              [ (idC +₁ τ _ ∘ (idC ⁂ extend idC) ∘ (idC ⁂ now)) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , i₂ ∘ τ _ ∘ (idC ⁂ extend idC) ]                   ≈⟨ sym ([]-cong₂ (pullˡ (+₁∘+₁ ○ +₁-cong₂ identity² assoc)) +₁∘i₂) ⟩
              [ (idC +₁ τ _ ∘ (idC ⁂ extend idC)) ∘ (idC +₁ (idC ⁂ now)) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , (idC +₁ τ _ ∘ (idC ⁂ extend idC)) ∘ i₂ ] ≈⟨ sym ∘[] ⟩
              (idC +₁ τ _ ∘ (idC ⁂ extend idC)) ∘ [ (idC +₁ (idC ⁂ now)) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , i₂ ]                                     ≈⟨ sym (refl⟩∘⟨ (elimʳ (IsIso.isoˡ isIsoˡ))) ⟩
              (idC +₁ τ _ ∘ (idC ⁂ extend idC)) ∘ [ (idC +₁ (idC ⁂ now)) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , i₂ ] ∘ distributeˡ⁻¹ ∘ distributeˡ       ∎
            tri₂ : (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ [ out , i₂ ∘ extend idC ]) ≈ (idC +₁ τ _ ∘ (idC ⁂ extend idC)) ∘ [ (idC +₁ (idC ⁂ now)) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , i₂ ] ∘ distributeˡ⁻¹
            tri₂ = Iso⇒Epi
              (IsIso.iso isIsoˡ)
              ((idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ [ out , i₂ ∘ extend idC ])) 
              ((idC +₁ τ _ ∘ (idC ⁂ extend idC)) ∘ [ (idC +₁ (idC ⁂ now)) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) , i₂ ] ∘ distributeˡ⁻¹) 
              (assoc ○ refl⟩∘⟨ assoc ○ tri₂' ○ sym-assoc ○ sym-assoc ○ assoc ⟩∘⟨refl)

        strength-assoc' : ∀ {X Y Z} → extend (now ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩) ∘ τ ((X × Y), Z) ≈ τ _ ∘ (idC ⁂ τ _) ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩
        strength-assoc' {X} {Y} {Z} = begin 
          extend (now ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩) ∘ τ _                                                                                                     ≈⟨ sym (Terminal.!-unique (coalgebras (X × Y × Z)) (record { f = extend (now ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩) ∘ τ _ ; commutes = begin 
            out ∘ extend (now ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩) ∘ τ _                                                                                        ≈⟨ pullˡ (extendlaw (now ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩)) ⟩ 
            ([ out ∘ now ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ , i₂ ∘ extend (now ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩) ] ∘ out) ∘ τ _                                ≈⟨ pullʳ (τ-law (X × Y , Z)) ⟩ 
            [ out ∘ now ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ , i₂ ∘ extend (now ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩) ] ∘ (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) ≈⟨ ([]-cong₂ (pullˡ unitlaw) refl) ⟩∘⟨refl ⟩ 
            (⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ +₁ extend (now ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩)) ∘ (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)                   ≈⟨ pullˡ +₁∘+₁ ⟩ 
            (⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ ∘ idC +₁ extend (now ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩) ∘ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)                      ≈⟨ (+₁-cong₂ id-comm (sym identityʳ)) ⟩∘⟨refl ⟩ 
            (idC ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ +₁ (extend (now ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩) ∘ τ _) ∘ idC) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)              ≈⟨ sym (pullˡ +₁∘+₁) ⟩ 
            (idC +₁ extend (now ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩) ∘ τ _) ∘ (⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ +₁ idC) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)             ∎ })) ⟩ 
          u (Terminal.! (coalgebras (X × Y × Z)) {A = record { A = (X × Y) × D₀ Z ; α = (⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ +₁ idC) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) }}) ≈⟨ Terminal.!-unique (coalgebras (X × Y × Z)) (record { f = τ _ ∘ (idC ⁂ τ _) ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ ; commutes = begin 
            out ∘ τ _ ∘ (idC ⁂ τ _) ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩                                                                              ≈⟨ pullˡ (τ-law (X , Y × Z)) ⟩ 
            ((idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)) ∘ (idC ⁂ τ _) ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩                                           ≈⟨ pullʳ (pullˡ (pullʳ ⁂∘⁂)) ⟩ 
            (idC +₁ τ _) ∘ (distributeˡ⁻¹ ∘ (idC ∘ idC ⁂ out ∘ τ _)) ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩                                             ≈⟨ refl⟩∘⟨ assoc ⟩ 
            (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ∘ idC ⁂ out ∘ τ _) ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩                                               ≈˘⟨ refl⟩∘⟨ (refl⟩∘⟨ (⁂-cong₂ identityʳ (assoc ○ sym (τ-law (Y , Z)))) ⟩∘⟨refl) ⟩ 
            (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ ((idC ∘ idC) ∘ idC ⁂ ((idC +₁ τ _) ∘ distributeˡ⁻¹) ∘ (idC ⁂ out)) ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩    ≈⟨ sym (refl⟩∘⟨ (pullʳ (pullˡ ⁂∘⁂ ○ pullˡ ⁂∘⁂))) ⟩ 
            (idC +₁ τ _) ∘ (distributeˡ⁻¹ ∘ (idC ⁂ (idC +₁ τ _))) ∘ (idC ⁂ distributeˡ⁻¹) ∘ (idC ⁂ (idC ⁂ out)) ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩  ≈⟨ refl⟩∘⟨ (refl⟩∘⟨ (refl⟩∘⟨ helper₁)) ⟩ 
            (idC +₁ τ _) ∘ (distributeˡ⁻¹ ∘ (idC ⁂ (idC +₁ τ _))) ∘ (idC ⁂ distributeˡ⁻¹) ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ ∘ (idC ⁂ out)          ≈⟨ refl⟩∘⟨ (helper₂ ⟩∘⟨refl) ⟩ 
            (idC +₁ τ _) ∘ ((idC +₁ (idC ⁂ τ (Y , Z))) ∘ distributeˡ⁻¹) ∘ (idC ⁂ distributeˡ⁻¹) ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ ∘ (idC ⁂ out)    ≈⟨ refl⟩∘⟨ (assoc ○ refl⟩∘⟨ sym assoc²') ⟩
            (idC +₁ τ _) ∘ (idC +₁ (idC ⁂ τ (Y , Z))) ∘ (distributeˡ⁻¹ ∘ (idC ⁂ distributeˡ⁻¹) ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩) ∘ (idC ⁂ out)    ≈⟨ pullˡ (+₁∘+₁ ○ +₁-cong₂ identity² refl) ⟩ 
            (idC +₁ τ _ ∘ (idC ⁂ τ (Y , Z))) ∘ (distributeˡ⁻¹ ∘ (idC ⁂ distributeˡ⁻¹) ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩) ∘ (idC ⁂ out)             ≈⟨ refl⟩∘⟨ (helper₃ ⟩∘⟨refl) ⟩ 
            (idC +₁ τ _ ∘ (idC ⁂ τ (Y , Z))) ∘ ((⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ +₁ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩) ∘ distributeˡ⁻¹) ∘ (idC ⁂ out) ≈⟨ assoc²'' ⟩
            ((idC +₁ τ _ ∘ (idC ⁂ τ _)) ∘ (⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ +₁ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩)) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)       ≈⟨ sym ((+₁-cong₂ refl (identityʳ ○ sym-assoc) ○ sym +₁∘+₁) ⟩∘⟨refl) ⟩ 
            (idC ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ +₁ (τ _ ∘ (idC ⁂ τ _) ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩) ∘ idC) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)    ≈⟨ sym (pullˡ +₁∘+₁) ⟩ 
            (idC +₁ τ _ ∘ (idC ⁂ τ _) ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩) ∘ (⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ +₁ idC) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)   ∎ }) ⟩
          τ _ ∘ (idC ⁂ τ _) ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩                                                                                                      ∎
          where
            helper₁ : (idC ⁂ (idC ⁂ out)) ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ ≈ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ ∘ (idC {X × Y} ⁂ out {Z})
            helper₁ = begin 
              (idC ⁂ (idC ⁂ out)) ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩                                       ≈⟨ ⁂∘⟨⟩ ⟩ 
              ⟨ idC ∘ π₁ ∘ π₁ , (idC ⁂ out) ∘ ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩                                         ≈⟨ ⟨⟩-cong₂ identityˡ ⁂∘⟨⟩ ⟩ 
              ⟨ π₁ ∘ π₁ , ⟨ idC ∘ π₂ ∘ π₁ , out ∘ π₂ ⟩ ⟩                                                 ≈⟨ ⟨⟩-cong₂ refl (⟨⟩-cong₂ identityˡ refl) ⟩ 
              ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , out ∘ π₂ ⟩ ⟩                                                       ≈⟨ sym (⟨⟩-cong₂ refl (⟨⟩-cong₂ (refl⟩∘⟨ identityˡ) refl)) ⟩ 
              ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ idC ∘ π₁ , out ∘ π₂ ⟩ ⟩                                                 ≈⟨ sym (⟨⟩-cong₂ (refl⟩∘⟨ identityˡ) (⟨⟩-cong₂ (pullʳ π₁∘⁂) π₂∘⁂)) ⟩ 
              ⟨ π₁ ∘ idC ∘ π₁ , ⟨ (π₂ ∘ π₁) ∘ (idC {X × Y} ⁂ out {Z}) , π₂ ∘ (idC {X × Y} ⁂ out {Z}) ⟩ ⟩ ≈⟨ sym (⟨⟩-cong₂ (pullʳ project₁) ⟨⟩∘) ⟩ 
              ⟨ (π₁ ∘ π₁) ∘ (idC {X × Y} ⁂ out {Z}) , ⟨ π₂ ∘ π₁ , π₂ ⟩ ∘ (idC {X × Y} ⁂ out {Z}) ⟩       ≈⟨ sym ⟨⟩∘ ⟩ 
              ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ ∘ (idC {X × Y} ⁂ out {Z})                                   ∎
            helper₂ : distributeˡ⁻¹ ∘ (idC ⁂ (idC +₁ τ _)) ≈ (idC +₁ (idC ⁂ τ (Y , Z))) ∘ distributeˡ⁻¹
            helper₂ = sym (distributeˡ⁻¹-natural idC idC (τ _)) ○ (+₁-cong₂ (⟨⟩-unique id-comm id-comm) refl) ⟩∘⟨refl
            helper₃ : distributeˡ⁻¹ ∘ (idC ⁂ distributeˡ⁻¹) ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ ≈ (⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ +₁ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩) ∘ distributeˡ⁻¹
            helper₃ = sym (begin 
              (⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ +₁ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩) ∘ distributeˡ⁻¹                                                                                                                                  ≈⟨ introˡ (IsIso.isoˡ isIsoˡ) ⟩ 
              (distributeˡ⁻¹ ∘ distributeˡ) ∘ (⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ +₁ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩) ∘ distributeˡ⁻¹                                                                                                  ≈⟨ pullʳ (pullˡ []∘+₁) ⟩
              distributeˡ⁻¹ ∘ [ (idC ⁂ i₁) ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ , (idC ⁂ i₂) ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ ] ∘ distributeˡ⁻¹                                                                                       ≈⟨ refl⟩∘⟨ (([]-cong₂ ⁂∘⟨⟩ ⁂∘⟨⟩) ⟩∘⟨refl) ⟩
              distributeˡ⁻¹ ∘ [ ⟨ idC ∘ π₁ ∘ π₁ , i₁ ∘ ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ , ⟨ idC ∘ π₁ ∘ π₁ , i₂ ∘ ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ ] ∘ distributeˡ⁻¹                                                                                           ≈⟨ refl⟩∘⟨ []-cong₂ (⟨⟩-cong₂ identityˡ (pushˡ (sym distributeˡ⁻¹-i₁))) (⟨⟩-cong₂ identityˡ ((pushˡ (sym distributeˡ⁻¹-i₂)))) ⟩∘⟨refl ⟩
              distributeˡ⁻¹ ∘ [ ⟨ π₁ ∘ π₁ , distributeˡ⁻¹ ∘ (idC ⁂ i₁) ∘ ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ , ⟨ π₁ ∘ π₁ , distributeˡ⁻¹ ∘ (idC ⁂ i₂) ∘ ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ ] ∘ distributeˡ⁻¹                                                       ≈⟨ sym (refl⟩∘⟨ []-cong₂ (⟨⟩-cong₂ refl (refl⟩∘⟨ (⟨⟩-cong₂ (refl⟩∘⟨ identityˡ ○ sym identityˡ) refl ○ sym ⁂∘⟨⟩))) (⟨⟩-cong₂ refl (refl⟩∘⟨ (⟨⟩-cong₂ (refl⟩∘⟨ identityˡ ○ sym identityˡ) refl ○ sym ⁂∘⟨⟩))) ⟩∘⟨refl) ⟩
              distributeˡ⁻¹ ∘ [ ⟨ π₁ ∘ π₁ , distributeˡ⁻¹ ∘ ⟨ π₂ ∘ idC ∘ π₁ , i₁ ∘ π₂ ⟩ ⟩ , ⟨ π₁ ∘ π₁ , distributeˡ⁻¹ ∘ ⟨ π₂ ∘ idC ∘ π₁ , i₂ ∘ π₂ ⟩ ⟩ ] ∘ distributeˡ⁻¹                                                           ≈⟨ sym (refl⟩∘⟨ []-cong₂ (⟨⟩-cong₂ (identityˡ ○ refl⟩∘⟨ identityˡ) (refl⟩∘⟨ (⟨⟩-cong₂ (pullʳ π₁∘⁂) π₂∘⁂))) (⟨⟩-cong₂ (identityˡ ○ refl⟩∘⟨ identityˡ) (refl⟩∘⟨ (⟨⟩-cong₂ (pullʳ π₁∘⁂) π₂∘⁂))) ⟩∘⟨refl) ⟩
              distributeˡ⁻¹ ∘ [ ⟨ idC ∘ π₁ ∘ idC ∘ π₁ , distributeˡ⁻¹ ∘ ⟨ (π₂ ∘ π₁) ∘ (idC ⁂ i₁) , π₂ ∘ (idC ⁂ i₁) ⟩ ⟩ , ⟨ idC ∘ π₁ ∘ idC ∘ π₁ , distributeˡ⁻¹ ∘ ⟨ (π₂ ∘ π₁) ∘ (idC ⁂ i₂) , π₂ ∘ (idC ⁂ i₂) ⟩ ⟩ ] ∘ distributeˡ⁻¹ ≈⟨ sym (refl⟩∘⟨ []-cong₂ (⟨⟩-cong₂ (pullʳ (pullʳ π₁∘⁂)) (pullʳ ⟨⟩∘)) (⟨⟩-cong₂ (pullʳ (pullʳ π₁∘⁂)) (pullʳ ⟨⟩∘)) ⟩∘⟨refl) ⟩
              distributeˡ⁻¹ ∘ [ ⟨ (idC ∘ π₁ ∘ π₁) ∘ (idC ⁂ i₁) , (distributeˡ⁻¹ ∘ ⟨ π₂ ∘ π₁ , π₂ ⟩) ∘ (idC ⁂ i₁) ⟩ , ⟨ (idC ∘ π₁ ∘ π₁) ∘ (idC ⁂ i₂) , (distributeˡ⁻¹ ∘ ⟨ π₂ ∘ π₁ , π₂ ⟩) ∘ (idC ⁂ i₂) ⟩ ] ∘ distributeˡ⁻¹         ≈⟨ sym (refl⟩∘⟨ []-cong₂ ⟨⟩∘ ⟨⟩∘ ⟩∘⟨refl) ⟩
              distributeˡ⁻¹ ∘ [ (⟨ idC ∘ π₁ ∘ π₁ , distributeˡ⁻¹ ∘ ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ ∘ (idC ⁂ i₁)) , (⟨ idC ∘ π₁ ∘ π₁ , distributeˡ⁻¹ ∘ ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩ ∘ (idC  ⁂ i₂)) ] ∘ distributeˡ⁻¹                                      ≈˘⟨ pullʳ (pullˡ ∘[]) ⟩
              (distributeˡ⁻¹ ∘ ⟨ idC ∘ π₁ ∘ π₁ , distributeˡ⁻¹ ∘ ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩) ∘ distributeˡ ∘ distributeˡ⁻¹                                                                                                                ≈⟨ sym (introʳ (IsIso.isoʳ isIsoˡ)) ⟩
              distributeˡ⁻¹ ∘ ⟨ idC ∘ π₁ ∘ π₁ , distributeˡ⁻¹ ∘ ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩                                                                                                                                                ≈⟨ sym (refl⟩∘⟨ ⁂∘⟨⟩) ⟩
              distributeˡ⁻¹ ∘ (idC ⁂ distributeˡ⁻¹) ∘ ⟨ π₁ ∘ π₁ , ⟨ π₂ ∘ π₁ , π₂ ⟩ ⟩                                                                                                                                              ∎)
-- {- ⁂ ⊤ ○ 
-- Diagram for identityˡ':
-- https://q.uiver.app/#q=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
-- -}
        commute' : ∀ {P₁ : Category.Obj (CProduct C C)} {P₂ : Category.Obj (CProduct C C)} (fg : _[_,_] (CProduct C C) P₁ P₂) 
          → τ P₂ ∘ ((proj₁ fg) ⁂ extend (now ∘ (proj₂ fg))) ≈ extend (now ∘ ((proj₁ fg) ⁂ (proj₂ fg))) ∘ τ P₁
        commute' {(U , V)} {(W , X)} (f , g) = begin 
          τ _ ∘ (f ⁂ extend (now ∘ g)) ≈⟨ sym (!-unique (record { f = τ _ ∘ (f ⁂ extend (now ∘ g)) ; commutes = begin 
            out ∘ τ (W , X) ∘ (f ⁂ extend (now ∘ g))                                                       ≈⟨ pullˡ (τ-law (W , X)) ⟩ 
            ((idC +₁ τ (W , X)) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)) ∘ (f ⁂ extend (now ∘ g))                    ≈⟨ pullʳ (pullʳ ⁂∘⁂) ⟩ 
            (idC +₁ τ (W , X)) ∘ distributeˡ⁻¹ ∘ (idC ∘ f ⁂ out ∘ extend (now ∘ g))                        ≈⟨ refl⟩∘⟨ (refl⟩∘⟨ (⁂-cong₂ identityˡ (extendlaw (now ∘ g)))) ⟩ 
            (idC +₁ τ (W , X)) ∘ distributeˡ⁻¹ ∘ (f ⁂ [ out ∘ now ∘ g , i₂ ∘ extend (now ∘ g) ] ∘ out)     ≈⟨ refl⟩∘⟨ (refl⟩∘⟨ (⁂-cong₂ refl (([]-cong₂ (pullˡ unitlaw) refl) ⟩∘⟨refl))) ⟩ 
            (idC +₁ τ (W , X)) ∘ distributeˡ⁻¹ ∘ (f ⁂ [ i₁ ∘ g , i₂ ∘ extend (now ∘ g) ] ∘ out)            ≈⟨ refl⟩∘⟨ (refl⟩∘⟨ (⁂-cong₂ (sym identityʳ) refl)) ⟩ 
            (idC +₁ τ (W , X)) ∘ distributeˡ⁻¹ ∘ (f ∘ idC ⁂ (g +₁ extend (now ∘ g)) ∘ out)                 ≈⟨ sym (pullʳ (pullʳ ⁂∘⁂)) ⟩ 
            ((idC +₁ τ (W , X)) ∘ distributeˡ⁻¹ ∘ (f ⁂ (g +₁ extend (now ∘ g)))) ∘ (idC ⁂ out)             ≈⟨ (refl⟩∘⟨ (sym (distributeˡ⁻¹-natural f g (extend (now ∘ g))))) ⟩∘⟨refl ⟩ 
            ((idC +₁ τ (W , X)) ∘ ((f ⁂ g) +₁ (f ⁂ extend (now ∘ g))) ∘ distributeˡ⁻¹) ∘ (idC ⁂ out)       ≈⟨ (pullˡ +₁∘+₁) ⟩∘⟨refl ⟩ 
            ((idC ∘ (f ⁂ g) +₁ τ (W , X) ∘ (f ⁂ extend (now ∘ g))) ∘ distributeˡ⁻¹) ∘ (idC ⁂ out)          ≈⟨ sym (((+₁-cong₂ refl identityʳ) ⟩∘⟨refl) ⟩∘⟨refl) ⟩ 
            ((idC ∘ (f ⁂ g) +₁ (τ (W , X) ∘ (f ⁂ extend (now ∘ g))) ∘ idC) ∘ distributeˡ⁻¹) ∘ (idC ⁂ out)  ≈⟨ sym (pullˡ (pullˡ +₁∘+₁)) ⟩ 
            (idC +₁ (τ (W , X) ∘ (f ⁂ extend (now ∘ g)))) ∘ ((f ⁂ g +₁ idC) ∘ distributeˡ⁻¹) ∘ (idC ⁂ out) ∎ })) ⟩ 
          u !                          ≈⟨ !-unique (record { f = extend (now ∘ (f ⁂ g)) ∘ τ _ ; commutes = begin 
            out ∘ extend (now ∘ (f ⁂ g)) ∘ τ _                                                                ≈⟨ pullˡ (extendlaw (now ∘ (f ⁂ g))) ⟩ 
            ([ out ∘ now ∘ (f ⁂ g) , i₂ ∘ extend (now ∘ (f ⁂ g)) ] ∘ out) ∘ τ (U , V)                         ≈⟨ (([]-cong₂ (pullˡ unitlaw) refl) ⟩∘⟨refl) ⟩∘⟨refl ⟩
            ([ i₁ ∘ (f ⁂ g) , i₂ ∘ extend (now ∘ (f ⁂ g)) ] ∘ out) ∘ τ (U , V)                                ≈⟨ pullʳ (τ-law (U , V)) ⟩
            ((f ⁂ g) +₁ (extend (now ∘ (f ⁂ g)))) ∘ (idC +₁ τ _) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out)                ≈⟨ sym-assoc ○ sym-assoc ⟩
            ((((f ⁂ g) +₁ (extend (now ∘ (f ⁂ g)))) ∘ (idC +₁ τ _)) ∘ distributeˡ⁻¹) ∘ (idC ⁂ out)            ≈⟨ (+₁∘+₁ ⟩∘⟨refl) ⟩∘⟨refl ⟩
            ((((f ⁂ g) ∘ idC) +₁ (extend (now ∘ (f ⁂ g)) ∘ τ (U , V))) ∘ distributeˡ⁻¹) ∘ (idC ⁂ out)         ≈⟨ sym (((+₁-cong₂ id-comm-sym identityʳ) ⟩∘⟨refl) ⟩∘⟨refl) ⟩
            (((idC ∘ (f ⁂ g)) +₁ ((extend (now ∘ (f ⁂ g)) ∘ τ (U , V)) ∘ idC)) ∘ distributeˡ⁻¹) ∘ (idC ⁂ out) ≈⟨ sym (pullˡ (pullˡ +₁∘+₁)) ⟩
            (idC +₁ (extend (now ∘ (f ⁂ g)) ∘ τ (U , V))) ∘ (((f ⁂ g) +₁ idC) ∘ distributeˡ⁻¹) ∘ (idC ⁂ out)  ∎ }) ⟩
          extend (now ∘ (f ⁂ g)) ∘ τ _ ∎
          where
            open Terminal (coalgebras (W × X))
            alg' : F-Coalgebra ((W × X) +-)
            alg' = record { A = U × D₀ V ; α = ((f ⁂ g) +₁ idC) ∘ distributeˡ⁻¹ ∘ (idC ⁂ out) }

    strongMonad : StrongMonad monoidal
    strongMonad = record { M = monad ; strength = strength }