-- Lean 4 proof step from Mathlib.CategoryTheory.Enriched.FunctorCategory (goal)
Goal:
V : Type u₁
inst : Category.{v₁, u₁} V
inst_1 : MonoidalCategory.{v₁, u₁} V
C : Type u₂
inst_2 : Category.{v₂, u₂} C
J : Type u₃
inst_3 : Category.{v₃, u₃} J
K : Type u₄
inst_4 : Category.{v₄, u₄} K
inst_5 : EnrichedOrdinaryCategory.{v₁, v₂, u₂, u₁} V C
F₁ F₂ F₃ F₄ : Functor.{v₃, v₂, u₃, u₂} J C
inst_6 : HasEnrichedHom.{v₁, v₂, v₃, u₁, u₂, u₃} V F₁ F₂
inst_7 : HasEnrichedHom.{v₁, v₂, v₃, u₁, u₂, u₃} V F₂ F₃
inst_8 : HasEnrichedHom.{v₁, v₂, v₃, u₁, u₂, u₃} V F₁ F₃
i j : J
f : i ⟶ j
this :
  Eq.{v₁ + 1} (α :=
    end_.{v₃, v₁, u₃, u₁} (diagram V F₂ F₃) ⟶
      @EnrichedCategory.Hom.{v₁, u₁, u₂} _ inst inst_1 _ _ (F₂.obj i) (F₃.obj j))
    (end_.π (diagram V F₂ F₃) i ≫
      CategoryStruct.comp (Y :=
        tensorObj.{v₁, u₁} (𝒞 := inst) (@EnrichedCategory.Hom.{v₁, u₁, u₂} _ inst inst_1 _ _ (F₂.obj i) (F₃.obj i))
          (tensorUnit.{v₁, u₁} V (𝒞 := inst)))
        (Iso.inv (ρ_ (@EnrichedCategory.Hom.{v₁, u₁, u₂} _ inst inst_1 _ _ (F₂.obj i) (F₃.obj i))))
        (CategoryStruct.comp (Y :=
          tensorObj.{v₁, u₁} (𝒞 := inst) (@EnrichedCategory.Hom.{v₁, u₁, u₂} _ inst inst_1 _ _ (F₂.obj i) (F₃.obj i))
            (@EnrichedCategory.Hom.{v₁, u₁, u₂} _ inst inst_1 _ _ (F₃.obj i) (F₃.obj j)))
          (@EnrichedCategory.Hom.{v₁, u₁, u₂} _ inst inst_1 _ _ (F₂.obj i) (F₃.obj i) ◁
            (eHomEquiv V : (a : F₃.obj i ⟶ F₃.obj j) → 𝟙_ V ⟶ F₃.obj i ⟶[V] F₃.obj j) (F₃.map f))
          (eComp V (F₂.obj i) (F₃.obj i) (F₃.obj j))))
    (end_.π (diagram V F₂ F₃) j ≫
      CategoryStruct.comp (Y :=
        tensorObj.{v₁, u₁} (𝒞 := inst) (tensorUnit.{v₁, u₁} V (𝒞 := inst))
          (@EnrichedCategory.Hom.{v₁, u₁, u₂} _ inst inst_1 _ _ (F₂.obj j) (F₃.obj j)))
        (Iso.inv (λ_ (@EnrichedCategory.Hom.{v₁, u₁, u₂} _ inst inst_1 _ _ (F₂.obj j) (F₃.obj j))))
        (CategoryStruct.comp (Y :=
          tensorObj.{v₁, u₁} (𝒞 := inst) (@EnrichedCategory.Hom.{v₁, u₁, u₂} _ inst inst_1 _ _ (F₂.obj i) (F₂.obj j))
            (@EnrichedCategory.Hom.{v₁, u₁, u₂} _ inst inst_1 _ _ (F₂.obj j) (F₃.obj j)))
          ((eHomEquiv V : (a : F₂.obj i ⟶ F₂.obj j) → 𝟙_ V ⟶ F₂.obj i ⟶[V] F₂.obj j) (F₂.map f) ▷
            @EnrichedCategory.Hom.{v₁, u₁, u₂} _ inst inst_1 _ _ (F₂.obj j) (F₃.obj j))
          (eComp V (F₂.obj i) (F₂.obj j) (F₃.obj j))))
| CategoryStruct.comp (Y := @EnrichedCategory.Hom.{v₁, u₁, u₂} _ inst inst_1 _ _ (F₁.obj i) (F₃.obj i))
    ((end_.π (diagram V F₁ F₂) i ⊗ₘ end_.π (diagram V F₂ F₃) i) ≫ eComp V (F₁.obj i) (F₂.obj i) (F₃.obj i))
    (CategoryStruct.comp (Y :=
      tensorObj.{v₁, u₁} (𝒞 := inst) (@EnrichedCategory.Hom.{v₁, u₁, u₂} _ inst inst_1 _ _ (F₁.obj i) (F₃.obj i))
        (tensorUnit.{v₁, u₁} V (𝒞 := inst)))
      (Iso.inv (ρ_ (@EnrichedCategory.Hom.{v₁, u₁, u₂} _ inst inst_1 _ _ (F₁.obj i) (F₃.obj i))))
      (CategoryStruct.comp (Y :=
        tensorObj.{v₁, u₁} (𝒞 := inst) (@EnrichedCategory.Hom.{v₁, u₁, u₂} _ inst inst_1 _ _ (F₁.obj i) (F₃.obj i))
          (@EnrichedCategory.Hom.{v₁, u₁, u₂} _ inst inst_1 _ _ (F₃.obj i) (F₃.obj j)))
        (@EnrichedCategory.Hom.{v₁, u₁, u₂} _ inst inst_1 _ _ (F₁.obj i) (F₃.obj i) ◁
          (eHomEquiv V : (a : F₃.obj i ⟶ F₃.obj j) → 𝟙_ V ⟶ F₃.obj i ⟶[V] F₃.obj j) (F₃.map f))
        (eComp V (F₁.obj i) (F₃.obj i) (F₃.obj j))))
have rw : end_.π (diagram V F₁ F₂) i ▷ enrichedHom.{v₁, v₂, v₃, u₁, u₂, u₃} V F₂ F₃ ≫
  @EnrichedCategory.Hom.{v₁, u₁, u₂} _ inst inst_1 _ _ (F₁.obj i) (F₂.obj i) ◁ end_.π (diagram V F₂ F₃) i ≫
    eComp V (F₁.obj i) (F₂.obj i) (F₃.obj i) ≫
      CategoryStruct.comp (Y :=
        tensorObj.{v₁, u₁} (𝒞 := inst) (@EnrichedCategory.Hom.{v₁, u₁, u₂} _ inst inst_1 _ _ (F₁.obj i) (F₃.obj i))
          (tensorUnit.{v₁, u₁} V (𝒞 := inst)))
        (Iso.inv (ρ_ (@EnrichedCategory.Hom.{v₁, u₁, u₂} _ inst inst_1 _ _ (F₁.obj i) (F₃.obj i))))
        (CategoryStruct.comp (Y :=
          tensorObj.{v₁, u₁} (𝒞 := inst) (@EnrichedCategory.Hom.{v₁, u₁, u₂} _ inst inst_1 _ _ (F₁.obj i) (F₃.obj i))
            (@EnrichedCategory.Hom.{v₁, u₁, u₂} _ inst inst_1 _ _ (F₃.obj i) (F₃.obj j)))
          (@EnrichedCategory.Hom.{v₁, u₁, u₂} _ inst inst_1 _ _ (F₁.obj i) (F₃.obj i) ◁
            (eHomEquiv V : (a : F₃.obj i ⟶ F₃.obj j) → 𝟙_ V ⟶ F₃.obj i ⟶[V] F₃.obj j) (F₃.map f))
          (eComp V (F₁.obj i) (F₃.obj i) (F₃.obj j)))
Tactic: rw [assoc, tensorHom_def_assoc]