-- Lean 4 proof step from Mathlib.CategoryTheory.Monoidal.Closed.Ideal (goal)
Goal:
C : Type u₁
D : Type u₂
inst : Category.{v₁, u₁} C
inst_1 : Category.{v₁, u₂} D
i : Functor.{v₁, v₁, u₂, u₁} D C
inst_2 : CartesianMonoidalCategory.{v₁, u₁} C
inst_3 : Reflective i
inst_4 : MonoidalClosed.{v₁, u₁} C
inst_5 : CartesianMonoidalCategory.{v₁, u₂} D
inst_6 : ExponentialIdeal i
inst_7 : BraidedCategory.{v₁, u₁} C
A B : C
rw :
  Eq (α :=
    tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B) ⟶
      tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B))
    ((bijection i A B (tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B)) :
          (a : (reflector i).obj (A ⊗ B) ⟶ (reflector i).obj A ⊗ (reflector i).obj B) →
            (reflector i).obj A ⊗ (reflector i).obj B ⟶ (reflector i).obj A ⊗ (reflector i).obj B)
        (𝟙 ((reflector i).obj (tensorObj.{v₁, u₁} (𝒞 := inst) A B)) ≫ prodComparison (reflector i) A B) :
      tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B) ⟶
        tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B))
    (𝟙 (tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B)) :
      tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B) ⟶
        tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B))
rw₁ :
  Eq (α :=
    tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B) ⟶
      tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B))
    ((bijection i A B (tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B)) :
          (a : (reflector i).obj (A ⊗ B) ⟶ (reflector i).obj A ⊗ (reflector i).obj B) →
            (reflector i).obj A ⊗ (reflector i).obj B ⟶ (reflector i).obj A ⊗ (reflector i).obj B)
        (prodComparison (reflector i) A B) :
      tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B) ⟶
        tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B))
    (𝟙 (tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B)) :
      tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B) ⟶
        tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B))
rw₂ :
  Eq (α :=
    tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B) ⟶
      tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B))
    ((bijection i A B (tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B)) :
          (a : (reflector i).obj (A ⊗ B) ⟶ (reflector i).obj A ⊗ (reflector i).obj B) →
            (reflector i).obj A ⊗ (reflector i).obj B ⟶ (reflector i).obj A ⊗ (reflector i).obj B)
        ((Equiv.symm (bijection i A B (tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B))) :
              (a : (reflector i).obj A ⊗ (reflector i).obj B ⟶ (reflector i).obj A ⊗ (reflector i).obj B) →
                (reflector i).obj (A ⊗ B) ⟶ (reflector i).obj A ⊗ (reflector i).obj B)
            (𝟙 (tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B))) :
          (reflector i).obj (tensorObj.{v₁, u₁} (𝒞 := inst) A B) ⟶
            tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B)) :
      tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B) ⟶
        tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B))
    (𝟙 (tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B)) :
      tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B) ⟶
        tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B))
⊢ Eq (α :=
    tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B) ⟶
      tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B))
    (CategoryStruct.comp (Y := (reflector i).obj (tensorObj.{v₁, u₁} (𝒞 := inst) A B))
        ((bijection i A B ((reflector i).obj (tensorObj.{v₁, u₁} (𝒞 := inst) A B)) :
            (a : (reflector i).obj (A ⊗ B) ⟶ (reflector i).obj (A ⊗ B)) →
              (reflector i).obj A ⊗ (reflector i).obj B ⟶ (reflector i).obj (A ⊗ B))
          (𝟙 ((reflector i).obj (tensorObj.{v₁, u₁} (𝒞 := inst) A B))))
        (prodComparison (reflector i) A B) :
      tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B) ⟶
        tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B))
    (𝟙 (tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B)) :
      tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B) ⟶
        tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B))
have rw₃ : Eq.{v₁ + 1} (α :=
  tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B) ⟶
    tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B))
  (𝟙 (tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B)))
  (𝟙 (tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B)))
Tactic: rw [← bijection_natural, Category.id_comp, ← bijection_symm_apply_id,
        Equiv.apply_symm_apply]