-- Lean 4 proof step from Mathlib.CategoryTheory.Enriched.Ordinary.Basic (goal)
Goal:
V : Type u'
inst : Category.{v', u'} V
inst_1 : MonoidalCategory.{v', u'} V
C : Type u
inst_2 : Category.{v, u} C
inst_3 : EnrichedOrdinaryCategory.{v', v, u, u'} V C
W_1 : Type u''
inst_4 : Category.{v'', u''} W_1
inst_5 : MonoidalCategory.{v'', u''} W_1
F_1 : Functor.{v', v'', u', u''} V W_1
inst_6 : F_1.LaxMonoidal
W : Type u''
inst_7 : Category.{v'', u''} W
inst_8 : MonoidalCategory.{v'', u''} W
F : Functor.{v', v'', u', u''} V W
inst_9 : F.LaxMonoidal
D : Type u
inst_10 : EnrichedCategory.{v', u', u} V D
e :
  (v : V) →
    Equiv.{v' + 1, v'' + 1} (tensorUnit.{v', u'} V (𝒞 := inst) ⟶ v) (tensorUnit.{v'', u''} W (𝒞 := inst_7) ⟶ F.obj v)
h :
  ∀ (v : V) (f : tensorUnit.{v', u'} V (𝒞 := inst) ⟶ v),
    Eq.{v'' + 1} (α := tensorUnit.{v'', u''} W (𝒞 := inst_7) ⟶ F.obj v)
      ((e v : (a : 𝟙_ V ⟶ v) → 𝟙_ W ⟶ F.obj v) f : tensorUnit.{v'', u''} W (𝒞 := inst_7) ⟶ F.obj v)
      (Functor.LaxMonoidal.ε F ≫ F.map f : tensorUnit.{v'', u''} W (𝒞 := inst_7) ⟶ F.obj v)
X Y Z : ForgetEnrichment.{v'', u'', u} W (TransportEnrichment F D)
f : X ⟶ Y
g : Y ⟶ Z
| Functor.LaxMonoidal.ε F ≫
    CategoryStruct.comp (Y :=
      F.obj (tensorObj.{v', u'} (𝒞 := inst) (tensorUnit.{v', u'} V (𝒞 := inst)) (tensorUnit.{v', u'} V (𝒞 := inst))))
      (F.map (Iso.inv (λ_ (tensorUnit.{v', u'} V (𝒞 := inst)))))
      (F.map
        ((Equiv.symm (e (@Hom.{v', u', u} _ inst inst_1 _ _ (ForgetEnrichment.to W X) (ForgetEnrichment.to W Y))) :
              (a : 𝟙_ W ⟶ F.obj (ForgetEnrichment.to W X ⟶[V] ForgetEnrichment.to W Y)) →
                𝟙_ V ⟶ ForgetEnrichment.to W X ⟶[V] ForgetEnrichment.to W Y)
            (ForgetEnrichment.homTo W f) ⊗ₘ
          (Equiv.symm (e (@Hom.{v', u', u} _ inst inst_1 _ _ (ForgetEnrichment.to W Y) (ForgetEnrichment.to W Z))) :
              (a : 𝟙_ W ⟶ F.obj (ForgetEnrichment.to W Y ⟶[V] ForgetEnrichment.to W Z)) →
                𝟙_ V ⟶ ForgetEnrichment.to W Y ⟶[V] ForgetEnrichment.to W Z)
            (ForgetEnrichment.homTo W g)))
have a : CategoryStruct.comp (Y :=
  tensorObj.{v'', u''} (𝒞 := inst_7) (tensorUnit.{v'', u''} W (𝒞 := inst_7)) (tensorUnit.{v'', u''} W (𝒞 := inst_7)))
  (Iso.inv (λ_ (tensorUnit.{v'', u''} W (𝒞 := inst_7))))
  ((CategoryStruct.comp (Y := F.obj (tensorUnit.{v', u'} V (𝒞 := inst))) (Functor.LaxMonoidal.ε F)
        (F.map
          ((Equiv.symm (e (@Hom.{v', u', u} _ inst inst_1 _ _ (ForgetEnrichment.to W X) (ForgetEnrichment.to W Y))) :
              (a : 𝟙_ W ⟶ F.obj (ForgetEnrichment.to W X ⟶[V] ForgetEnrichment.to W Y)) →
                𝟙_ V ⟶ ForgetEnrichment.to W X ⟶[V] ForgetEnrichment.to W Y)
            (ForgetEnrichment.homTo W f))) ⊗ₘ
      CategoryStruct.comp (Y := F.obj (tensorUnit.{v', u'} V (𝒞 := inst))) (Functor.LaxMonoidal.ε F)
        (F.map
          ((Equiv.symm (e (@Hom.{v', u', u} _ inst inst_1 _ _ (ForgetEnrichment.to W Y) (ForgetEnrichment.to W Z))) :
              (a : 𝟙_ W ⟶ F.obj (ForgetEnrichment.to W Y ⟶[V] ForgetEnrichment.to W Z)) →
                𝟙_ V ⟶ ForgetEnrichment.to W Y ⟶[V] ForgetEnrichment.to W Z)
            (ForgetEnrichment.homTo W g)))) ≫
    Functor.LaxMonoidal.μ F (@Hom.{v', u', u} _ inst inst_1 _ _ (ForgetEnrichment.to W X) (ForgetEnrichment.to W Y))
      (@Hom.{v', u', u} _ inst inst_1 _ _ (ForgetEnrichment.to W Y) (ForgetEnrichment.to W Z)))
Tactic: rw [← Functor.LaxMonoidal.left_unitality_inv, Category.assoc, Category.assoc,
        ← Functor.LaxMonoidal.μ_natural, ← leftUnitor_inv_comp_tensorHom_assoc,
        tensorHom_comp_tensorHom_assoc]