-- Lean 4 proof step from Mathlib.CategoryTheory.Monoidal.DayConvolution (goal)
Goal:
C_1 : Type u₁
inst : Category.{v₁, u₁} C_1
V_1 : Type u₂
inst_1 : Category.{v₂, u₂} V_1
inst_2 : MonoidalCategory.{v₁, u₁} C_1
inst_3 : MonoidalCategory.{v₂, u₂} V_1
C : Type u₁
inst_4 : Category.{v₁, u₁} C
V : Type u₂
inst_5 : Category.{v₂, u₂} V
inst_6 : MonoidalCategory.{v₁, u₁} C
inst_7 : MonoidalCategory.{v₂, u₂} V
D : Type u₃
inst_8 : Category.{v₃, u₃} D
inst_9 : MonoidalCategoryStruct.{v₃, u₃} D
inst_10 : LawfulDayConvolutionMonoidalCategoryStruct.{v₁, v₂, v₃, u₁, u₂, u₃} C V D
inst_11 :
  ∀ (v : V) (d : C), Limits.PreservesColimitsOfShape (CostructuredArrow.{v₁, v₁, u₁, u₁} (tensor C) d) (tensorLeft v)
inst_12 :
  ∀ (v : V) (d : C), Limits.PreservesColimitsOfShape (CostructuredArrow.{v₁, v₁, u₁, u₁} (tensor C) d) (tensorRight v)
inst_13 :
  ∀ (v : V) (d : C),
    Limits.PreservesColimitsOfShape.{0, v₁, v₂, v₂, u₂, u₂}
      (CostructuredArrow (Functor.fromPUnit (tensorUnit.{v₁, u₁} C (𝒞 := inst_4))) d) (tensorLeft v)
inst_14 :
  ∀ (v : V) (d : C),
    Limits.PreservesColimitsOfShape.{0, v₁, v₂, v₂, u₂, u₂}
      (CostructuredArrow (Functor.fromPUnit (tensorUnit.{v₁, u₁} C (𝒞 := inst_4))) d) (tensorRight v)
inst_15 :
  ∀ (v : V) (d : C × C),
    Limits.PreservesColimitsOfShape
      (CostructuredArrow.{v₁, v₁, u₁, u₁} (Functor.prod (𝟭 C) (Functor.fromPUnit (tensorUnit.{v₁, u₁} C (𝒞 := inst_4))))
        d)
      (tensorRight v)
inst_16 :
  ∀ (v : V) (d : C × C),
    Limits.PreservesColimitsOfShape (CostructuredArrow.{v₁, v₁, u₁, u₁} (Functor.prod (tensor C) (𝟭 C)) d)
      (tensorRight v)
X₁ Y₁ Z₁ X₂ Y₂ Z₂ : D
x : X₁ ⟶ Y₁
x_1 : X₂ ⟶ Y₂
x_2 : Y₁ ⟶ Z₁
x_3 : Y₂ ⟶ Z₂
⊢ Eq (α :=
    Quiver.Hom (V := Functor.{v₁, v₂, u₁, u₂} (C × C) V)
      (Functor.obj (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) X₁ ⊠ Functor.obj (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) X₂)
      (tensor C ⋙ Functor.obj (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) (tensorObj.{v₃, u₃} (𝒞 := inst_8) Z₁ Z₂)))
    ((Functor.CorepresentableBy.homEquiv
            (corepresentableBy (Functor.obj (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) X₁)
              (Functor.obj (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) X₂)) :
          (a : (ι C V D).obj X₁ ⊛ (ι C V D).obj X₂ ⟶ (ι C V D).obj (Z₁ ⊗ Z₂)) →
            (ι C V D).obj X₁ ⊠ (ι C V D).obj X₂ ⟶ tensor C ⋙ (ι C V D).obj (Z₁ ⊗ Z₂))
        (CategoryStruct.comp (Y :=
          Functor.obj (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) (tensorObj.{v₃, u₃} (𝒞 := inst_8) Y₁ Y₂))
          (map (Functor.map (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) x)
            (Functor.map (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) x_1))
          (map (Functor.map (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) x_2)
            (Functor.map (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) x_3))) :
      Quiver.Hom (V := Functor.{v₁, v₂, u₁, u₂} (C × C) V)
        (Functor.obj (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) X₁ ⊠
          Functor.obj (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) X₂)
        (tensor C ⋙ Functor.obj (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) (tensorObj.{v₃, u₃} (𝒞 := inst_8) Z₁ Z₂)))
    ((Functor.CorepresentableBy.homEquiv
            (corepresentableBy (Functor.obj (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) X₁)
              (Functor.obj (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) X₂)) :
          (a : (ι C V D).obj X₁ ⊛ (ι C V D).obj X₂ ⟶ (ι C V D).obj (Z₁ ⊗ Z₂)) →
            (ι C V D).obj X₁ ⊠ (ι C V D).obj X₂ ⟶ tensor C ⋙ (ι C V D).obj (Z₁ ⊗ Z₂))
        (map
          (Functor.map (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) x ≫
            Functor.map (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) x_2)
          (Functor.map (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) x_1 ≫
            Functor.map (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) x_3)) :
      Quiver.Hom (V := Functor.{v₁, v₂, u₁, u₂} (C × C) V)
        (Functor.obj (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) X₁ ⊠
          Functor.obj (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) X₂)
        (tensor C ⋙ Functor.obj (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) (tensorObj.{v₃, u₃} (𝒞 := inst_8) Z₁ Z₂)))
have a : ∀ (fst snd : C),
  Eq.{v₂ + 1} (α :=
    Functor.obj.{v₁, v₂, u₁, u₂}
        (Functor.obj (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) X₁ ⊠
          Functor.obj (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) X₂)
        (fst, snd) ⟶
      Functor.obj.{v₁, v₂, u₁, u₂}
        (tensor C ⋙ Functor.obj (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) (tensorObj.{v₃, u₃} (𝒞 := inst_8) Z₁ Z₂))
        (fst, snd))
    (NatTrans.app
      ((Functor.CorepresentableBy.homEquiv
            (corepresentableBy (Functor.obj (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) X₁)
              (Functor.obj (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) X₂)) :
          (a : (ι C V D).obj X₁ ⊛ (ι C V D).obj X₂ ⟶ (ι C V D).obj (Z₁ ⊗ Z₂)) →
            (ι C V D).obj X₁ ⊠ (ι C V D).obj X₂ ⟶ tensor C ⋙ (ι C V D).obj (Z₁ ⊗ Z₂))
        (CategoryStruct.comp (Y :=
          Functor.obj (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) (tensorObj.{v₃, u₃} (𝒞 := inst_8) Y₁ Y₂))
          (map (Functor.map (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) x)
            (Functor.map (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) x_1))
          (map (Functor.map (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) x_2)
            (Functor.map (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) x_3))))
      (fst, snd))
    (NatTrans.app
      ((Functor.CorepresentableBy.homEquiv
            (corepresentableBy (Functor.obj (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) X₁)
              (Functor.obj (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) X₂)) :
          (a : (ι C V D).obj X₁ ⊛ (ι C V D).obj X₂ ⟶ (ι C V D).obj (Z₁ ⊗ Z₂)) →
            (ι C V D).obj X₁ ⊠ (ι C V D).obj X₂ ⟶ tensor C ⋙ (ι C V D).obj (Z₁ ⊗ Z₂))
        (map
          (Functor.map (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) x ≫
            Functor.map (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) x_2)
          (Functor.map (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) x_1 ≫
            Functor.map (@ι C _ V _ inst_6 inst_7 D _ inst_9 _) x_3)))
      (fst, snd))
Tactic: ext ⟨_, _⟩