-- Lean 4 proof step from Mathlib.CategoryTheory.Monoidal.Cartesian.Basic (goal)
Goal:
C : Type u
inst : Category.{v, u} C
ℬ : (X Y : C) → LimitCone.{0, 0, v, u} (pair X Y)
X₁ X₂ X₃ Y₁ Y₂ Y₃ : C
f₁ : X₁ ⟶ Y₁
f₂ : X₂ ⟶ Y₂
f₃ : X₃ ⟶ Y₃
⊢ Eq (α :=
    tensorObj ℬ (tensorObj ℬ X₁ X₂) X₃ ⟶
      Cone.pt (F := pair Y₁ (ℬ Y₂ Y₃).cone.pt) (LimitCone.cone (ℬ Y₁ (ℬ Y₂ Y₃).cone.pt)))
    (IsLimit.lift (LimitCone.isLimit (ℬ (tensorObj ℬ Y₁ Y₂) Y₃))
          (BinaryFan.mk (P := Cone.pt (F := pair (tensorObj ℬ X₁ X₂) X₃) (LimitCone.cone (ℬ (tensorObj ℬ X₁ X₂) X₃)))
            (CategoryStruct.comp (Y := tensorObj ℬ X₁ X₂)
                (NatTrans.app (Cone.π (LimitCone.cone (ℬ (tensorObj ℬ X₁ X₂) X₃))) { as := WalkingPair.left } :
                  Functor.obj.{0, v, 0, u}
                      (Functor.obj (Functor.const (Discrete WalkingPair))
                        (Cone.pt (F := pair (tensorObj ℬ X₁ X₂) X₃) (LimitCone.cone (ℬ (tensorObj ℬ X₁ X₂) X₃))))
                      { as := WalkingPair.left : Discrete WalkingPair } ⟶
                    Functor.obj.{0, v, 0, u} (pair (tensorObj ℬ X₁ X₂) X₃)
                      { as := WalkingPair.left : Discrete WalkingPair })
                ((ℬ Y₁ Y₂).isLimit.lift
                  (BinaryFan.mk (P := (ℬ X₁ X₂).cone.pt)
                    ((ℬ X₁ X₂).cone.π.app { as := WalkingPair.left } ≫ f₁ : (ℬ X₁ X₂).cone.pt ⟶ Y₁)
                    ((ℬ X₁ X₂).cone.π.app { as := WalkingPair.right } ≫ f₂ : (ℬ X₁ X₂).cone.pt ⟶ Y₂))) :
              Cone.pt (F := pair (tensorObj ℬ X₁ X₂) X₃) (LimitCone.cone (ℬ (tensorObj ℬ X₁ X₂) X₃)) ⟶
                tensorObj ℬ Y₁ Y₂)
            (NatTrans.app (Cone.π (LimitCone.cone (ℬ (tensorObj ℬ X₁ X₂) X₃))) { as := WalkingPair.right } ≫ f₃ :
              Cone.pt (F := pair (tensorObj ℬ X₁ X₂) X₃) (LimitCone.cone (ℬ (tensorObj ℬ X₁ X₂) X₃)) ⟶ Y₃)) ≫
        (BinaryFan.associatorOfLimitCone ℬ Y₁ Y₂ Y₃).hom :
      tensorObj ℬ (tensorObj ℬ X₁ X₂) X₃ ⟶
        Cone.pt (F := pair Y₁ (ℬ Y₂ Y₃).cone.pt) (LimitCone.cone (ℬ Y₁ (ℬ Y₂ Y₃).cone.pt)))
    ((BinaryFan.associatorOfLimitCone ℬ X₁ X₂ X₃).hom ≫
        IsLimit.lift (LimitCone.isLimit (ℬ Y₁ (tensorObj ℬ Y₂ Y₃)))
          (BinaryFan.mk (P := Cone.pt (F := pair X₁ (ℬ X₂ X₃).cone.pt) (LimitCone.cone (ℬ X₁ (ℬ X₂ X₃).cone.pt)))
            (NatTrans.app (Cone.π (LimitCone.cone (ℬ X₁ (ℬ X₂ X₃).cone.pt))) { as := WalkingPair.left } ≫ f₁ :
              Cone.pt (F := pair X₁ (ℬ X₂ X₃).cone.pt) (LimitCone.cone (ℬ X₁ (ℬ X₂ X₃).cone.pt)) ⟶ Y₁)
            (CategoryStruct.comp (Y := (ℬ X₂ X₃).cone.pt)
                (NatTrans.app (Cone.π (LimitCone.cone (ℬ X₁ (ℬ X₂ X₃).cone.pt))) { as := WalkingPair.right } :
                  Functor.obj.{0, v, 0, u}
                      (Functor.obj (Functor.const (Discrete WalkingPair))
                        (Cone.pt (F := pair X₁ (ℬ X₂ X₃).cone.pt) (LimitCone.cone (ℬ X₁ (ℬ X₂ X₃).cone.pt))))
                      { as := WalkingPair.right : Discrete WalkingPair } ⟶
                    Functor.obj.{0, v, 0, u} (pair X₁ (ℬ X₂ X₃).cone.pt)
                      { as := WalkingPair.right : Discrete WalkingPair })
                ((ℬ Y₂ Y₃).isLimit.lift
                  (BinaryFan.mk (P := (ℬ X₂ X₃).cone.pt)
                    ((ℬ X₂ X₃).cone.π.app { as := WalkingPair.left } ≫ f₂ : (ℬ X₂ X₃).cone.pt ⟶ Y₂)
                    ((ℬ X₂ X₃).cone.π.app { as := WalkingPair.right } ≫ f₃ : (ℬ X₂ X₃).cone.pt ⟶ Y₃))) :
              Cone.pt (F := pair X₁ (ℬ X₂ X₃).cone.pt) (LimitCone.cone (ℬ X₁ (ℬ X₂ X₃).cone.pt)) ⟶ tensorObj ℬ Y₂ Y₃)) :
      Cone.pt (F := pair (ℬ X₁ X₂).cone.pt X₃) (LimitCone.cone (ℬ (ℬ X₁ X₂).cone.pt X₃)) ⟶
        tensorObj ℬ Y₁ (tensorObj ℬ Y₂ Y₃))
have apply : ∀ (j : Discrete WalkingPair),
  Eq (α := tensorObj ℬ (tensorObj ℬ X₁ X₂) X₃ ⟶ Functor.obj.{0, v, 0, u} (pair Y₁ (ℬ Y₂ Y₃).cone.pt) j)
    (CategoryStruct.comp (Y := Cone.pt (F := pair Y₁ (ℬ Y₂ Y₃).cone.pt) (LimitCone.cone (ℬ Y₁ (ℬ Y₂ Y₃).cone.pt)))
        (IsLimit.lift (LimitCone.isLimit (ℬ (tensorObj ℬ Y₁ Y₂) Y₃))
            (BinaryFan.mk (P := Cone.pt (F := pair (tensorObj ℬ X₁ X₂) X₃) (LimitCone.cone (ℬ (tensorObj ℬ X₁ X₂) X₃)))
              (CategoryStruct.comp (Y := tensorObj ℬ X₁ X₂)
                  (NatTrans.app (Cone.π (LimitCone.cone (ℬ (tensorObj ℬ X₁ X₂) X₃))) { as := WalkingPair.left } :
                    Functor.obj.{0, v, 0, u}
                        (Functor.obj (Functor.const (Discrete WalkingPair))
                          (Cone.pt (F := pair (tensorObj ℬ X₁ X₂) X₃) (LimitCone.cone (ℬ (tensorObj ℬ X₁ X₂) X₃))))
                        { as := WalkingPair.left : Discrete WalkingPair } ⟶
                      Functor.obj.{0, v, 0, u} (pair (tensorObj ℬ X₁ X₂) X₃)
                        { as := WalkingPair.left : Discrete WalkingPair })
                  ((ℬ Y₁ Y₂).isLimit.lift
                    (BinaryFan.mk (P := (ℬ X₁ X₂).cone.pt)
                      ((ℬ X₁ X₂).cone.π.app { as := WalkingPair.left } ≫ f₁ : (ℬ X₁ X₂).cone.pt ⟶ Y₁)
                      ((ℬ X₁ X₂).cone.π.app { as := WalkingPair.right } ≫ f₂ : (ℬ X₁ X₂).cone.pt ⟶ Y₂))) :
                Cone.pt (F := pair (tensorObj ℬ X₁ X₂) X₃) (LimitCone.cone (ℬ (tensorObj ℬ X₁ X₂) X₃)) ⟶
                  tensorObj ℬ Y₁ Y₂)
              (NatTrans.app (Cone.π (LimitCone.cone (ℬ (tensorObj ℬ X₁ X₂) X₃))) { as := WalkingPair.right } ≫ f₃ :
                Cone.pt (F := pair (tensorObj ℬ X₁ X₂) X₃) (LimitCone.cone (ℬ (tensorObj ℬ X₁ X₂) X₃)) ⟶ Y₃)) ≫
          (BinaryFan.associatorOfLimitCone ℬ Y₁ Y₂ Y₃).hom)
        (NatTrans.app (Cone.π (LimitCone.cone (ℬ Y₁ (ℬ Y₂ Y₃).cone.pt))) j) :
      tensorObj ℬ (tensorObj ℬ X₁ X₂) X₃ ⟶ Functor.obj.{0, v, 0, u} (pair Y₁ (ℬ Y₂ Y₃).cone.pt) j)
    (CategoryStruct.comp (Y := Cone.pt (F := pair Y₁ (ℬ Y₂ Y₃).cone.pt) (LimitCone.cone (ℬ Y₁ (ℬ Y₂ Y₃).cone.pt)))
        ((BinaryFan.associatorOfLimitCone ℬ X₁ X₂ X₃).hom ≫
          IsLimit.lift (LimitCone.isLimit (ℬ Y₁ (tensorObj ℬ Y₂ Y₃)))
            (BinaryFan.mk (P := Cone.pt (F := pair X₁ (ℬ X₂ X₃).cone.pt) (LimitCone.cone (ℬ X₁ (ℬ X₂ X₃).cone.pt)))
              (NatTrans.app (Cone.π (LimitCone.cone (ℬ X₁ (ℬ X₂ X₃).cone.pt))) { as := WalkingPair.left } ≫ f₁ :
                Cone.pt (F := pair X₁ (ℬ X₂ X₃).cone.pt) (LimitCone.cone (ℬ X₁ (ℬ X₂ X₃).cone.pt)) ⟶ Y₁)
              (CategoryStruct.comp (Y := (ℬ X₂ X₃).cone.pt)
                  (NatTrans.app (Cone.π (LimitCone.cone (ℬ X₁ (ℬ X₂ X₃).cone.pt))) { as := WalkingPair.right } :
                    Functor.obj.{0, v, 0, u}
                        (Functor.obj (Functor.const (Discrete WalkingPair))
                          (Cone.pt (F := pair X₁ (ℬ X₂ X₃).cone.pt) (LimitCone.cone (ℬ X₁ (ℬ X₂ X₃).cone.pt))))
                        { as := WalkingPair.right : Discrete WalkingPair } ⟶
                      Functor.obj.{0, v, 0, u} (pair X₁ (ℬ X₂ X₃).cone.pt)
                        { as := WalkingPair.right : Discrete WalkingPair })
                  ((ℬ Y₂ Y₃).isLimit.lift
                    (BinaryFan.mk (P := (ℬ X₂ X₃).cone.pt)
                      ((ℬ X₂ X₃).cone.π.app { as := WalkingPair.left } ≫ f₂ : (ℬ X₂ X₃).cone.pt ⟶ Y₂)
                      ((ℬ X₂ X₃).cone.π.app { as := WalkingPair.right } ≫ f₃ : (ℬ X₂ X₃).cone.pt ⟶ Y₃))) :
                Cone.pt (F := pair X₁ (ℬ X₂ X₃).cone.pt) (LimitCone.cone (ℬ X₁ (ℬ X₂ X₃).cone.pt)) ⟶
                  tensorObj ℬ Y₂ Y₃)))
        (NatTrans.app (Cone.π (LimitCone.cone (ℬ Y₁ (ℬ Y₂ Y₃).cone.pt))) j) :
      tensorObj ℬ (tensorObj ℬ X₁ X₂) X₃ ⟶ Functor.obj.{0, v, 0, u} (pair Y₁ (ℬ Y₂ Y₃).cone.pt) j)
Tactic: apply (ℬ _ _).isLimit.hom_ext