-- Lean 4 proof step from Mathlib.CategoryTheory.Monoidal.Arrow (goal)
Goal:
C : Type u
inst : Category.{v, u} C
inst_1 : HasPushouts.{v, u} C
inst_2 : HasInitial.{v, u} C
inst_3 : CartesianMonoidalCategory.{v, u} C
inst_4 : MonoidalClosed.{v, u} C
inst_5 : BraidedCategory.{v, u} C
X Y : Arrow.{v, u} C
⊢ Eq (α :=
    obj.{v, v, u, u}
        (obj (curriedTensor C)
          (tensorObj (𝒞 := instCategoryArrow) X ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))).left)
        Y.right ⟶
      (tensorObj (𝒞 := instCategoryArrow) X Y).left)
    (CategoryStruct.comp (Y :=
      pushout (X :=
        obj.{v, v, u, u}
          (obj (curriedTensor C)
            (tensorObj (𝒞 := instCategoryArrow) X ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))).left)
          Y.left)
        (NatTrans.app
          (map (curriedTensor C)
            (tensorObj (𝒞 := instCategoryArrow) X ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))).hom)
          Y.left)
        (map
          (obj (curriedTensor C)
            (tensorObj (𝒞 := instCategoryArrow) X ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))).left)
          Y.hom))
      (pushout.inr
        (NatTrans.app
          (map (curriedTensor C)
            (tensorObj (𝒞 := instCategoryArrow) X ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))).hom)
          Y.left)
        (map
          (obj (curriedTensor C)
            (tensorObj (𝒞 := instCategoryArrow) X ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))).left)
          Y.hom))
      (Arrow.Hom.left (Iso.hom (α_ X ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow)) Y) ≫ X ◁ Iso.hom (λ_ Y))))
    (CategoryStruct.comp (Y :=
      pushout (X :=
        obj.{v, v, u, u}
          (obj (curriedTensor C)
            (tensorObj (𝒞 := instCategoryArrow) X ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))).left)
          Y.left)
        (NatTrans.app
          (map (curriedTensor C)
            (tensorObj (𝒞 := instCategoryArrow) X ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))).hom)
          Y.left)
        (map
          (obj (curriedTensor C)
            (tensorObj (𝒞 := instCategoryArrow) X ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))).left)
          Y.hom))
      (pushout.inr
        (NatTrans.app
          (map (curriedTensor C)
            (tensorObj (𝒞 := instCategoryArrow) X ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))).hom)
          Y.left)
        (map
          (obj (curriedTensor C)
            (tensorObj (𝒞 := instCategoryArrow) X ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))).left)
          Y.hom))
      (Arrow.Hom.left (Iso.hom (ρ_ X) ▷ Y)))
have h₀ : Eq (α :=
  obj.{v, v, u, u} (tensorRight Y.right)
      (obj.{v, v, u, u} (obj (curriedTensor C) X.right)
        (Arrow.left.{v, u} ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow)))) ⟶
    (tensorObj (𝒞 := instCategoryArrow) X Y).left)
  (CategoryStruct.comp (Y :=
    obj.{v, v, u, u} (tensorRight Y.right)
      (pushout (X :=
        obj.{v, v, u, u} (obj (curriedTensor C) X.left)
          (Arrow.left.{v, u} ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))))
        (NatTrans.app (map (curriedTensor C) X.hom)
          (Arrow.left.{v, u} ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))))
        (map (obj (curriedTensor C) X.left) (Arrow.hom ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))))))
    (map (tensorRight Y.right)
      (pushout.inl
        (NatTrans.app (map (curriedTensor C) X.hom)
          (Arrow.left.{v, u} ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))))
        (map (obj (curriedTensor C) X.left) (Arrow.hom ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))))))
    (CategoryStruct.comp (Y :=
      pushout (X :=
        obj.{v, v, u, u}
          (obj (curriedTensor C)
            (tensorObj (𝒞 := instCategoryArrow) X ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))).left)
          Y.left)
        (NatTrans.app
          (map (curriedTensor C)
            (tensorObj (𝒞 := instCategoryArrow) X ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))).hom)
          Y.left)
        (map
          (obj (curriedTensor C)
            (tensorObj (𝒞 := instCategoryArrow) X ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))).left)
          Y.hom))
      (pushout.inr
        (NatTrans.app
          (map (curriedTensor C)
            (tensorObj (𝒞 := instCategoryArrow) X ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))).hom)
          Y.left)
        (map
          (obj (curriedTensor C)
            (tensorObj (𝒞 := instCategoryArrow) X ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))).left)
          Y.hom))
      (Arrow.Hom.left (Iso.hom (α_ X ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow)) Y) ≫ X ◁ Iso.hom (λ_ Y)))))
  (CategoryStruct.comp (Y :=
    obj.{v, v, u, u} (tensorRight Y.right)
      (pushout (X :=
        obj.{v, v, u, u} (obj (curriedTensor C) X.left)
          (Arrow.left.{v, u} ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))))
        (NatTrans.app (map (curriedTensor C) X.hom)
          (Arrow.left.{v, u} ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))))
        (map (obj (curriedTensor C) X.left) (Arrow.hom ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))))))
    (map (tensorRight Y.right)
      (pushout.inl
        (NatTrans.app (map (curriedTensor C) X.hom)
          (Arrow.left.{v, u} ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))))
        (map (obj (curriedTensor C) X.left) (Arrow.hom ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))))))
    (CategoryStruct.comp (Y :=
      pushout (X :=
        obj.{v, v, u, u}
          (obj (curriedTensor C)
            (tensorObj (𝒞 := instCategoryArrow) X ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))).left)
          Y.left)
        (NatTrans.app
          (map (curriedTensor C)
            (tensorObj (𝒞 := instCategoryArrow) X ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))).hom)
          Y.left)
        (map
          (obj (curriedTensor C)
            (tensorObj (𝒞 := instCategoryArrow) X ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))).left)
          Y.hom))
      (pushout.inr
        (NatTrans.app
          (map (curriedTensor C)
            (tensorObj (𝒞 := instCategoryArrow) X ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))).hom)
          Y.left)
        (map
          (obj (curriedTensor C)
            (tensorObj (𝒞 := instCategoryArrow) X ((𝟙_ (Arrow.{v, u} C)) (𝒞 := instCategoryArrow))).left)
          Y.hom))
      (Arrow.Hom.left (Iso.hom (ρ_ X) ▷ Y))))
Tactic: apply ((tensorRight _).map_isPushout (IsPushout.of_hasPushout _ _)).hom_ext