-- 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
⊢ tensorHom ℬ (tensorHom ℬ f₁ f₂) f₃ ≫ (BinaryFan.associatorOfLimitCone ℬ Y₁ Y₂ Y₃).hom =
    (BinaryFan.associatorOfLimitCone ℬ X₁ X₂ X₃).hom ≫ tensorHom ℬ f₁ (tensorHom ℬ f₂ f₃)
have dsimp : 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))
Tactic: dsimp [tensorHom]