/-- The isomorphism between `n ⊗ X` and `normalize X n` is natural (in both `X` and `n`, but
    naturality in `n` is trivial and was "proved" in `normalizeIsoAux`). This is the real heart
    of our proof of the coherence theorem. -/
def normalizeIso : tensorFunc C ≅ normalize' C :=
  NatIso.ofComponents (normalizeIsoAux C)
    (by -- porting note: the proof has been mostly rewritten
      rintro X Y f
      induction' f using Quotient.recOn with f; swap; rfl
      induction' f with _ X₁ X₂ X₃ _ _ _ _ _ _ _ _ _ _ _ _ h₁ h₂ X₁ X₂ Y₁ Y₂ f g h₁ h₂
      · simp only [mk_id, Functor.map_id, Category.comp_id, Category.id_comp]
      · ext n
        dsimp
        rw [mk_α_hom, NatTrans.comp_app, NatTrans.comp_app]
        dsimp [NatIso.ofComponents, normalizeMapAux, whiskeringRight, whiskerRight, Functor.comp]
        simp only [comp_tensor_id, associator_conjugation, tensor_id,
          Category.comp_id]
        simp only [← Category.assoc]
        congr 4
        simp only [Category.assoc, ← cancel_epi (𝟙 (inclusionObj n.as) ⊗ (α_ X₁ X₂ X₃).inv),
          pentagon_inv_assoc (inclusionObj n.as) X₁ X₂ X₃,
          tensor_inv_hom_id_assoc, tensor_id, Category.id_comp, Iso.inv_hom_id,
          Category.comp_id]
      · ext n
        dsimp
        rw [mk_α_inv, NatTrans.comp_app, NatTrans.comp_app]
        dsimp [NatIso.ofComponents, normalizeMapAux, whiskeringRight, whiskerRight, Functor.comp]
        simp only [Category.assoc, comp_tensor_id, tensor_id, Category.comp_id,
          pentagon_inv_assoc, ← associator_inv_naturality_assoc]
        rfl
      · ext n
        dsimp [Functor.comp]
        rw [mk_l_hom, NatTrans.comp_app, NatTrans.comp_app]
        dsimp [NatIso.ofComponents, normalizeMapAux, whiskeringRight, whiskerRight, Functor.comp]
        simp only [triangle_assoc_comp_right_assoc, Category.assoc, Category.comp_id]
        rfl
      · ext n
        dsimp [Functor.comp]
        rw [mk_l_inv, NatTrans.comp_app, NatTrans.comp_app]
        dsimp [NatIso.ofComponents, normalizeMapAux, whiskeringRight, whiskerRight, Functor.comp]
        simp only [triangle_assoc_comp_left_inv_assoc, inv_hom_id_tensor_assoc, tensor_id,
          Category.id_comp, Category.comp_id]
        rfl
      · ext n
        dsimp
        rw [mk_ρ_hom, NatTrans.comp_app, NatTrans.comp_app]
        dsimp [NatIso.ofComponents, normalizeMapAux, whiskeringRight, whiskerRight, Functor.comp]
        simp only [← (Iso.inv_comp_eq _).2 (rightUnitor_tensor _ _), Category.assoc,
          ← rightUnitor_naturality, Category.comp_id]; rfl
      · ext n
        dsimp
        rw [mk_ρ_inv, NatTrans.comp_app, NatTrans.comp_app]
        dsimp [NatIso.ofComponents, normalizeMapAux, whiskeringRight, whiskerRight, Functor.comp]
        simp only [← (Iso.eq_comp_inv _).1 (rightUnitor_tensor_inv _ _), rightUnitor_conjugation,
          Category.assoc, Iso.hom_inv_id_assoc, Iso.inv_hom_id_assoc, Iso.inv_hom_id,
          Discrete.functor, Category.comp_id, Function.comp]
      · rw [mk_comp, Functor.map_comp, Functor.map_comp, Category.assoc, h₂, reassoc_of% h₁]
      · ext ⟨n⟩
        

Proof state:
case f.tensor.w.h.mk
C : Type u
X Y X₁ X₂ Y₁ Y₂ : F C
f : X₁ ⟶ᵐ Y₁
g : X₂ ⟶ᵐ Y₂
h₁ : (tensorFunc C).map ⟦f⟧ ≫ (normalizeIsoAux C Y₁).hom = (normalizeIsoAux C X₁).hom ≫ (normalize' C).map ⟦f⟧
h₂ : (tensorFunc C).map ⟦g⟧ ≫ (normalizeIsoAux C Y₂).hom = (normalizeIsoAux C X₂).hom ≫ (normalize' C).map ⟦g⟧
n : NormalMonoidalObject C
⊢ ((tensorFunc C).map ⟦Hom.tensor f g⟧ ≫ (normalizeIsoAux C (tensor Y₁ Y₂)).hom).app { as := n } =
    ((normalizeIsoAux C (tensor X₁ X₂)).hom ≫ (normalize' C).map ⟦Hom.tensor f g⟧).app { as := n }

Next tactic:
replace h₁ := NatTrans.congr_app h₁ ⟨n⟩