-- Lean 4 proof step from Mathlib.CategoryTheory.Monoidal.DayConvolution (goal)
Goal:
C : Type u₁
inst : Category.{v₁, u₁} C
V : Type u₂
inst_1 : Category.{v₂, u₂} V
inst_2 : MonoidalCategory.{v₁, u₁} C
inst_3 : MonoidalCategory.{v₂, u₂} V
F G : Functor.{v₁, v₂, u₁, u₂} C V
inst_4 : DayConvolution F G
H : Functor.{v₁, v₂, u₁, u₂} C V
inst_5 : DayConvolution G H
inst_6 : DayConvolution F (@convolution _ _ _ _ inst_2 inst_3 G H _)
inst_7 : DayConvolution (@convolution _ _ _ _ inst_2 inst_3 F G _) H
inst_8 :
  ∀ (v : V) (d : C), Limits.PreservesColimitsOfShape (CostructuredArrow.{v₁, v₁, u₁, u₁} (tensor C) d) (tensorLeft v)
inst_9 :
  ∀ (v : V) (d : C), Limits.PreservesColimitsOfShape (CostructuredArrow.{v₁, v₁, u₁, u₁} (tensor C) d) (tensorRight v)
F' G' H' : Functor.{v₁, v₂, u₁, u₂} C V
inst_10 : DayConvolution F' G'
inst_11 : DayConvolution G' H'
inst_12 : DayConvolution F' (@convolution _ _ _ _ inst_2 inst_3 G' H' _)
inst_13 : DayConvolution (@convolution _ _ _ _ inst_2 inst_3 F' G' _) H'
f : F ⟶ F'
g : G ⟶ G'
h : H ⟶ H'
x : (C × C) × C
⊢ Eq (α :=
    tensorObj.{v₂, u₂} (𝒞 := inst_1) (tensorObj.{v₂, u₂} (𝒞 := inst_1) (F.obj x.1.1) (G.obj x.1.2)) (H.obj x.2) ⟶
      (@convolution _ _ _ _ inst_2 inst_3 F' (@convolution _ _ _ _ inst_2 inst_3 G' H' _) _).obj
        (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) x.1.1 x.1.2) x.2))
    (CategoryStruct.comp (Y :=
      tensorObj.{v₂, u₂} (𝒞 := inst_1)
        ((@convolution _ _ _ _ inst_2 inst_3 F G _).obj (tensorObj.{v₁, u₁} (𝒞 := inst) x.1.1 x.1.2)) (H.obj x.2))
      (NatTrans.app (unit F G) x.1 ▷ H.obj x.2)
      (CategoryStruct.comp (Y :=
        tensorObj.{v₂, u₂} (𝒞 := inst_1)
          ((@convolution _ _ _ _ inst_2 inst_3 F' G' _).obj (tensorObj.{v₁, u₁} (𝒞 := inst) x.1.1 x.1.2)) (H'.obj x.2))
        (NatTrans.app (map f g) (tensorObj.{v₁, u₁} (𝒞 := inst) x.1.1 x.1.2) ⊗ₘ h.app x.2)
        (CategoryStruct.comp (Y :=
          (@convolution _ _ _ _ inst_2 inst_3 (@convolution _ _ _ _ inst_2 inst_3 F' G' _) H' _).obj
            (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) x.1.1 x.1.2) x.2))
          (NatTrans.app (unit (@convolution _ _ _ _ inst_2 inst_3 F' G' _) H')
            (tensorObj.{v₁, u₁} (𝒞 := inst) x.1.1 x.1.2, x.2))
          (NatTrans.app (Iso.hom (associator F' G' H'))
            (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) x.1.1 x.1.2) x.2)))))
    (CategoryStruct.comp (Y :=
      tensorObj.{v₂, u₂} (𝒞 := inst_1)
        ((@convolution _ _ _ _ inst_2 inst_3 F G _).obj (tensorObj.{v₁, u₁} (𝒞 := inst) x.1.1 x.1.2)) (H.obj x.2))
      (NatTrans.app (unit F G) x.1 ▷ H.obj x.2)
      (CategoryStruct.comp (Y :=
        (@convolution _ _ _ _ inst_2 inst_3 (@convolution _ _ _ _ inst_2 inst_3 F G _) H _).obj
          (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) x.1.1 x.1.2) x.2))
        (NatTrans.app (unit (@convolution _ _ _ _ inst_2 inst_3 F G _) H)
          (tensorObj.{v₁, u₁} (𝒞 := inst) x.1.1 x.1.2, x.2))
        (CategoryStruct.comp (Y :=
          (@convolution _ _ _ _ inst_2 inst_3 F (@convolution _ _ _ _ inst_2 inst_3 G H _) _).obj
            (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) x.1.1 x.1.2) x.2))
          (NatTrans.app (Iso.hom (associator F G H))
            (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) x.1.1 x.1.2) x.2))
          (NatTrans.app (map f (map g h))
            (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) x.1.1 x.1.2) x.2)))))
have a : Eq (α :=
  tensorObj.{v₂, u₂} (𝒞 := inst_1) (tensorObj.{v₂, u₂} (𝒞 := inst_1) (F.obj x.1.1) (G.obj x.1.2)) (H.obj x.2) ⟶
    (@convolution _ _ _ _ inst_2 inst_3 F' (@convolution _ _ _ _ inst_2 inst_3 G' H' _) _).obj
      (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) x.1.1 x.1.2) x.2))
  (CategoryStruct.comp (Y :=
    tensorObj.{v₂, u₂} (𝒞 := inst_1)
      ((@convolution _ _ _ _ inst_2 inst_3 F G _).obj (tensorObj.{v₁, u₁} (𝒞 := inst) x.1.1 x.1.2)) (H.obj x.2))
    (NatTrans.app (unit F G) x.1 ▷ H.obj x.2)
    (CategoryStruct.comp (Y :=
      tensorObj.{v₂, u₂} (𝒞 := inst_1)
        ((@convolution _ _ _ _ inst_2 inst_3 F' G' _).obj (tensorObj.{v₁, u₁} (𝒞 := inst) x.1.1 x.1.2)) (H'.obj x.2))
      (NatTrans.app (map f g) (tensorObj.{v₁, u₁} (𝒞 := inst) x.1.1 x.1.2) ⊗ₘ h.app x.2)
      (CategoryStruct.comp (Y :=
        (@convolution _ _ _ _ inst_2 inst_3 (@convolution _ _ _ _ inst_2 inst_3 F' G' _) H' _).obj
          (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) x.1.1 x.1.2) x.2))
        (NatTrans.app (unit (@convolution _ _ _ _ inst_2 inst_3 F' G' _) H')
          (tensorObj.{v₁, u₁} (𝒞 := inst) x.1.1 x.1.2, x.2))
        (NatTrans.app (Iso.hom (associator F' G' H'))
          (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) x.1.1 x.1.2) x.2)))))
  (Iso.hom (α_ (F.obj x.1.1) (G.obj x.1.2) (H.obj x.2)) ≫
    CategoryStruct.comp (Y :=
      tensorObj.{v₂, u₂} (𝒞 := inst_1) (F.obj x.1.1)
        ((@convolution _ _ _ _ inst_2 inst_3 G H _).obj (obj.{v₁, v₁, u₁, u₁} (tensor C) (x.1.2, x.2))))
      (F.obj x.1.1 ◁ NatTrans.app (unit G H) (x.1.2, x.2) :
        tensorObj.{v₂, u₂} (𝒞 := inst_1) (F.obj x.1.1) (tensorObj.{v₂, u₂} (𝒞 := inst_1) (G.obj x.1.2) (H.obj x.2)) ⟶
          tensorObj.{v₂, u₂} (𝒞 := inst_1) (F.obj x.1.1)
            ((@convolution _ _ _ _ inst_2 inst_3 G H _).obj (obj.{v₁, v₁, u₁, u₁} (tensor C) (x.1.2, x.2))))
      (NatTrans.app (unit F (@convolution _ _ _ _ inst_2 inst_3 G H _))
            (x.1.1, tensorObj.{v₁, u₁} (𝒞 := inst) x.1.2 x.2) ≫
          (@convolution _ _ _ _ inst_2 inst_3 F (@convolution _ _ _ _ inst_2 inst_3 G H _) _).map
              (Iso.inv (α_ x.1.1 x.1.2 x.2)) ≫
            NatTrans.app (map f (map g h))
              (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) x.1.1 x.1.2) x.2) :
        tensorObj.{v₂, u₂} (𝒞 := inst_1) (F.obj x.1.1)
            ((@convolution _ _ _ _ inst_2 inst_3 G H _).obj (obj.{v₁, v₁, u₁, u₁} (tensor C) (x.1.2, x.2))) ⟶
          (@convolution _ _ _ _ inst_2 inst_3 F' (@convolution _ _ _ _ inst_2 inst_3 G' H' _) _).obj
            (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) x.1.1 x.1.2) x.2)))
Tactic: rw [associator_hom_unit_unit_assoc]