-- Lean 4 proof step from Mathlib.CategoryTheory.Localization.Monoidal.Basic (hypothesis)
Goal:
C : Type u_1
D : Type u_2
inst : Category.{v_1, u_1} C
inst_1 : Category.{v_2, u_2} D
L : Functor.{v_1, v_2, u_1, u_2} C D
W : MorphismProperty.{v_1, u_1} C
inst_2 : MonoidalCategory.{v_1, u_1} C
inst_3 : W.IsMonoidal
inst_4 : L.IsLocalization W
unit : D
ε : L.obj (tensorUnit.{v_1, u_1} C (𝒞 := inst)) ≅ unit
X Y : LocalizedMonoidal L W ε
X' : C
e₁ : Functor.obj L' X' ≅ X
Y' : C
e₂ : Functor.obj L' Y' ≅ Y
h₁ :
  Eq (α :=
    Quiver.Hom (V := LocalizedMonoidal L W ε)
      (tensorObj.{v_2, u_2} (𝒞 := instCategoryLocalizedMonoidal L W ε)
        (tensorObj.{v_2, u_2} (𝒞 := instCategoryLocalizedMonoidal L W ε) (Functor.obj L' X')
          (Functor.obj L' (tensorUnit.{v_1, u_1} C (𝒞 := inst))))
        (Functor.obj L' Y'))
      (tensorObj.{v_2, u_2} (𝒞 := instCategoryLocalizedMonoidal L W ε) (Functor.obj L' X')
        (Functor.obj L' (tensorObj.{v_1, u_1} (𝒞 := inst) (tensorUnit.{v_1, u_1} C (𝒞 := inst)) Y'))))
    (Iso.hom (α_ (Functor.obj L' X') (Functor.obj L' (tensorUnit.{v_1, u_1} C (𝒞 := inst))) (Functor.obj L' Y')) ≫
      (𝟙 (Functor.obj L' X') ⊗ₘ Iso.hom (μ L W ε (tensorUnit.{v_1, u_1} C (𝒞 := inst)) Y')))
    (((Iso.hom (μ L W ε X' (tensorUnit.{v_1, u_1} C (𝒞 := inst))) ⊗ₘ 𝟙 (Functor.obj L' Y')) ≫
        Iso.hom (μ L W ε (tensorObj.{v_1, u_1} (𝒞 := inst) X' (tensorUnit.{v_1, u_1} C (𝒞 := inst))) Y') ≫
          Functor.map L' (Iso.hom (α_ X' (tensorUnit.{v_1, u_1} C (𝒞 := inst)) Y')) ≫
            Iso.inv (μ L W ε X' (tensorObj.{v_1, u_1} (𝒞 := inst) (tensorUnit.{v_1, u_1} C (𝒞 := inst)) Y')) ≫
              (𝟙 (Functor.obj L' X') ⊗ₘ Iso.inv (μ L W ε (tensorUnit.{v_1, u_1} C (𝒞 := inst)) Y'))) ≫
      (𝟙 (Functor.obj L' X') ⊗ₘ Iso.hom (μ L W ε (tensorUnit.{v_1, u_1} C (𝒞 := inst)) Y')))
⊢ Eq (α :=
    tensorObj.{v_2, u_2} (𝒞 := instCategoryLocalizedMonoidal L W ε)
        (tensorObj.{v_2, u_2} (𝒞 := instCategoryLocalizedMonoidal L W ε) X
          (tensorUnit.{v_2, u_2} (LocalizedMonoidal L W ε) (𝒞 := instCategoryLocalizedMonoidal L W ε)))
        Y ⟶
      tensorObj.{v_2, u_2} (𝒞 := instCategoryLocalizedMonoidal L W ε) X Y)
    (Iso.hom (α_ X (tensorUnit.{v_2, u_2} (LocalizedMonoidal L W ε) (𝒞 := instCategoryLocalizedMonoidal L W ε)) Y) ≫
      X ◁ Iso.hom (λ_ Y))
    (Iso.hom (ρ_ X) ▷ Y)
have h₂ : Eq (α :=
  Quiver.Hom (V := LocalizedMonoidal L W ε)
    (tensorObj.{v_2, u_2} (𝒞 := instCategoryLocalizedMonoidal L W ε)
      (tensorObj.{v_2, u_2} (𝒞 := instCategoryLocalizedMonoidal L W ε) (Functor.obj L' X')
        (Functor.obj L' (tensorUnit.{v_1, u_1} C (𝒞 := inst))))
      (Functor.obj L' Y'))
    (Functor.obj L'
      (tensorObj.{v_1, u_1} (𝒞 := inst) X'
        (tensorObj.{v_1, u_1} (𝒞 := inst) (tensorUnit.{v_1, u_1} C (𝒞 := inst)) Y'))))
  (Iso.hom (α_ (Functor.obj L' X') (Functor.obj L' (tensorUnit.{v_1, u_1} C (𝒞 := inst))) (Functor.obj L' Y')) ≫
    Functor.obj L' X' ◁ Iso.hom (μ L W ε (tensorUnit.{v_1, u_1} C (𝒞 := inst)) Y') ≫
      Iso.hom (μ L W ε X' (tensorObj.{v_1, u_1} (𝒞 := inst) (tensorUnit.{v_1, u_1} C (𝒞 := inst)) Y')))
  ((Iso.hom (μ L W ε X' (tensorUnit.{v_1, u_1} C (𝒞 := inst))) ⊗ₘ 𝟙 (Functor.obj L' Y')) ≫
    Iso.hom (μ L W ε (tensorObj.{v_1, u_1} (𝒞 := inst) X' (tensorUnit.{v_1, u_1} C (𝒞 := inst))) Y') ≫
      Functor.map L' (Iso.hom (α_ X' (tensorUnit.{v_1, u_1} C (𝒞 := inst)) Y')))
Tactic: simp only [assoc, id_tensorHom, ← whiskerLeft_comp,
    Iso.inv_hom_id, whiskerLeft_id, comp_id, Iso.inv_hom_id,
    ← cancel_mono (μ L W ε X' (𝟙_ C ⊗ Y')).hom] at h₁
Cleared hypotheses: h₁