-- 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
X X' : D
f : (reflector i).obj (tensorObj.{v₁, u₁} (𝒞 := inst) A B) ⟶ X
g : X ⟶ X'
⊢ Eq (α := tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B) ⟶ X')
    (i.fullyFaithfulOfReflective.preimage
      (CategoryStruct.comp (Y :=
        tensorObj.{v₁, u₁} (𝒞 := inst) (i.obj ((reflector i).obj A)) (i.obj ((reflector i).obj B)))
        (prodComparison i ((reflector i).obj A) ((reflector i).obj B))
        (uncurry
            ((unitCompPartialBijective B ⋯ :
                (a : B ⟶ i.obj ((reflector i).obj A) ⟹ i.obj X') →
                  i.obj ((reflector i).obj B) ⟶ i.obj ((reflector i).obj A) ⟹ i.obj X')
              (curry
                (Iso.inv (β_ B (i.obj ((reflector i).obj A))) ≫
                  uncurry
                      ((unitCompPartialBijective A ⋯ :
                          (a : A ⟶ B ⟹ i.obj X') → i.obj ((reflector i).obj A) ⟶ B ⟹ i.obj X')
                        (curry
                          (Iso.inv (β_ A B) ≫
                            (Adjunction.homEquiv (reflectorAdjunction i) (tensorObj.{v₁, u₁} (𝒞 := inst) A B) X' :
                                  (a : (reflector i).obj (A ⊗ B) ⟶ X') → A ⊗ B ⟶ i.obj X')
                                (f ≫ g) ≫
                              𝟙 (i.obj X')))) ≫
                    𝟙 (i.obj X')))) ≫
          𝟙 (i.obj X'))))
    (i.fullyFaithfulOfReflective.preimage
        (CategoryStruct.comp (Y :=
          tensorObj.{v₁, u₁} (𝒞 := inst) (i.obj ((reflector i).obj A)) (i.obj ((reflector i).obj B)))
          (prodComparison i ((reflector i).obj A) ((reflector i).obj B))
          (uncurry
              ((unitCompPartialBijective B ⋯ :
                  (a : B ⟶ i.obj ((reflector i).obj A) ⟹ i.obj X) →
                    i.obj ((reflector i).obj B) ⟶ i.obj ((reflector i).obj A) ⟹ i.obj X)
                (curry
                  (Iso.inv (β_ B (i.obj ((reflector i).obj A))) ≫
                    uncurry
                        ((unitCompPartialBijective A ⋯ :
                            (a : A ⟶ B ⟹ i.obj X) → i.obj ((reflector i).obj A) ⟶ B ⟹ i.obj X)
                          (curry
                            (Iso.inv (β_ A B) ≫
                              (Adjunction.homEquiv (reflectorAdjunction i) (tensorObj.{v₁, u₁} (𝒞 := inst) A B) X :
                                    (a : (reflector i).obj (A ⊗ B) ⟶ X) → A ⊗ B ⟶ i.obj X)
                                  f ≫
                                𝟙 (i.obj X)))) ≫
                      𝟙 (i.obj X)))) ≫
            𝟙 (i.obj X))) ≫
      g)
have a : Eq (α := i.obj (tensorObj.{v₁, u₂} (𝒞 := inst_1) ((reflector i).obj A) ((reflector i).obj B)) ⟶ i.obj X')
  (i.map
    (i.fullyFaithfulOfReflective.preimage
      (CategoryStruct.comp (Y :=
        tensorObj.{v₁, u₁} (𝒞 := inst) (i.obj ((reflector i).obj A)) (i.obj ((reflector i).obj B)))
        (prodComparison i ((reflector i).obj A) ((reflector i).obj B))
        (uncurry
            ((unitCompPartialBijective B sorry :
                (a : B ⟶ i.obj ((reflector i).obj A) ⟹ i.obj X') →
                  i.obj ((reflector i).obj B) ⟶ i.obj ((reflector i).obj A) ⟹ i.obj X')
              (curry
                (Iso.inv (β_ B (i.obj ((reflector i).obj A))) ≫
                  uncurry
                      ((unitCompPartialBijective A sorry :
                          (a : A ⟶ B ⟹ i.obj X') → i.obj ((reflector i).obj A) ⟶ B ⟹ i.obj X')
                        (curry
                          (Iso.inv (β_ A B) ≫
                            (Adjunction.homEquiv (reflectorAdjunction i) (tensorObj.{v₁, u₁} (𝒞 := inst) A B) X' :
                                  (a : (reflector i).obj (A ⊗ B) ⟶ X') → A ⊗ B ⟶ i.obj X')
                                (f ≫ g) ≫
                              𝟙 (i.obj X')))) ≫
                    𝟙 (i.obj X')))) ≫
          𝟙 (i.obj X')))))
  (i.map
    (i.fullyFaithfulOfReflective.preimage
        (CategoryStruct.comp (Y :=
          tensorObj.{v₁, u₁} (𝒞 := inst) (i.obj ((reflector i).obj A)) (i.obj ((reflector i).obj B)))
          (prodComparison i ((reflector i).obj A) ((reflector i).obj B))
          (uncurry
              ((unitCompPartialBijective B sorry :
                  (a : B ⟶ i.obj ((reflector i).obj A) ⟹ i.obj X) →
                    i.obj ((reflector i).obj B) ⟶ i.obj ((reflector i).obj A) ⟹ i.obj X)
                (curry
                  (Iso.inv (β_ B (i.obj ((reflector i).obj A))) ≫
                    uncurry
                        ((unitCompPartialBijective A sorry :
                            (a : A ⟶ B ⟹ i.obj X) → i.obj ((reflector i).obj A) ⟶ B ⟹ i.obj X)
                          (curry
                            (Iso.inv (β_ A B) ≫
                              (Adjunction.homEquiv (reflectorAdjunction i) (tensorObj.{v₁, u₁} (𝒞 := inst) A B) X :
                                    (a : (reflector i).obj (A ⊗ B) ⟶ X) → A ⊗ B ⟶ i.obj X)
                                  f ≫
                                𝟙 (i.obj X)))) ≫
                      𝟙 (i.obj X)))) ≫
            𝟙 (i.obj X))) ≫
      g))
Tactic: apply i.map_injective