-- Lean 4 proof step from Mathlib.Algebra.Category.ModuleCat.Presheaf.Monoidal (goal)
Goal:
C : Type u_1
inst : Category.{v_1, u_1} C
R : CategoryTheory.Functor.{v_1, u, u_1, u + 1} Cᵒᵖ CommRingCat
M :
  PresheafOfModules
    (R ⋙
      forget₂ CommRingCat (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
        CommRingCat.carrier) RingCat (FD := fun (R S : RingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CD :=
        RingCat.carrier))
X Y : Cᵒᵖ
f : X ⟶ Y
⊢ M.map f ≫
      CategoryTheory.Functor.map
        (ModuleCat.restrictScalars
          (RingCat.Hom.hom
            (CategoryTheory.Functor.map
              (R ⋙
                forget₂ CommRingCat (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
                  CommRingCat.carrier) RingCat (FD := fun (R S : RingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CD :=
                  RingCat.carrier))
              f)))
        (Iso.hom
          ((fun (x : Cᵒᵖ) ↦
              (Iso.symm (λ_ (M.obj x)) :
                M.obj x ≅
                  MonoidalCategoryStruct.tensorObj (𝒞 :=
                    ModuleCat.moduleCategory
                      (↑(Functor.obj
                            (R ⋙
                              forget₂ CommRingCat (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u))
                                (CC := CommRingCat.carrier) RingCat (FD := fun (R S : RingCat.{u}) ↦
                                (↑R : Type u) →+* (↑S : Type u)) (CD := RingCat.carrier))
                            x) :
                        Type u))
                    ((𝟙_
                        (ModuleCat
                          (↑(Functor.obj
                                (R ⋙
                                  forget₂ CommRingCat (FC := fun (R S : CommRingCat.{u}) ↦
                                    (↑R : Type u) →+* (↑S : Type u)) (CC := CommRingCat.carrier) RingCat (FD :=
                                    fun (R S : RingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CD := RingCat.carrier))
                                x) :
                            Type u)))
                      (𝒞 :=
                      ModuleCat.moduleCategory
                        (↑(Functor.obj
                              (R ⋙
                                forget₂ CommRingCat (FC := fun (R S : CommRingCat.{u}) ↦
                                  (↑R : Type u) →+* (↑S : Type u)) (CC := CommRingCat.carrier) RingCat (FD :=
                                  fun (R S : RingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CD := RingCat.carrier))
                              x) :
                          Type u)))
                    (M.obj x)))
            Y)) =
    Iso.hom
        ((fun (x : Cᵒᵖ) ↦
            (Iso.symm (λ_ (M.obj x)) :
              M.obj x ≅
                MonoidalCategoryStruct.tensorObj (𝒞 :=
                  ModuleCat.moduleCategory
                    (↑(Functor.obj
                          (R ⋙
                            forget₂ CommRingCat (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u))
                              (CC := CommRingCat.carrier) RingCat (FD := fun (R S : RingCat.{u}) ↦
                              (↑R : Type u) →+* (↑S : Type u)) (CD := RingCat.carrier))
                          x) :
                      Type u))
                  ((𝟙_
                      (ModuleCat
                        (↑(Functor.obj
                              (R ⋙
                                forget₂ CommRingCat (FC := fun (R S : CommRingCat.{u}) ↦
                                  (↑R : Type u) →+* (↑S : Type u)) (CC := CommRingCat.carrier) RingCat (FD :=
                                  fun (R S : RingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CD := RingCat.carrier))
                              x) :
                          Type u)))
                    (𝒞 :=
                    ModuleCat.moduleCategory
                      (↑(Functor.obj
                            (R ⋙
                              forget₂ CommRingCat (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u))
                                (CC := CommRingCat.carrier) RingCat (FD := fun (R S : RingCat.{u}) ↦
                                (↑R : Type u) →+* (↑S : Type u)) (CD := RingCat.carrier))
                            x) :
                        Type u)))
                  (M.obj x)))
          X) ≫
      (Monoidal.tensorObj
            (unit
              (R ⋙
                forget₂ CommRingCat (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
                  CommRingCat.carrier) RingCat (FD := fun (R S : RingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CD :=
                  RingCat.carrier)))
            M).map
        f
have hf : ∀ (m : (↑(M.obj X) : Type u)),
  Eq (α :=
    (↑(Functor.obj
          (ModuleCat.restrictScalars
            (RingCat.Hom.hom
              (CategoryTheory.Functor.map
                (R ⋙
                  forget₂ CommRingCat (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
                    CommRingCat.carrier) RingCat (FD := fun (R S : RingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u))
                    (CD := RingCat.carrier))
                f)))
          ((Monoidal.tensorObj
                (unit
                  (R ⋙
                    forget₂ CommRingCat (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
                      CommRingCat.carrier) RingCat (FD := fun (R S : RingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u))
                      (CD := RingCat.carrier)))
                M).obj
            Y)) :
      Type u))
    ((ModuleCat.Hom.hom
            (M.map f ≫
              CategoryTheory.Functor.map
                (ModuleCat.restrictScalars
                  (RingCat.Hom.hom
                    (CategoryTheory.Functor.map
                      (R ⋙
                        forget₂ CommRingCat (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
                          CommRingCat.carrier) RingCat (FD := fun (R S : RingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u))
                          (CD := RingCat.carrier))
                      f)))
                (Iso.hom
                  ((fun (x : Cᵒᵖ) ↦
                      (Iso.symm (λ_ (M.obj x)) :
                        M.obj x ≅
                          MonoidalCategoryStruct.tensorObj (𝒞 :=
                            ModuleCat.moduleCategory
                              (↑(Functor.obj
                                    (R ⋙
                                      forget₂ CommRingCat (FC := fun (R S : CommRingCat.{u}) ↦
                                        (↑R : Type u) →+* (↑S : Type u)) (CC := CommRingCat.carrier) RingCat (FD :=
                                        fun (R S : RingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CD :=
                                        RingCat.carrier))
                                    x) :
                                Type u))
                            ((𝟙_
                                (ModuleCat
                                  (↑(Functor.obj
                                        (R ⋙
                                          forget₂ CommRingCat (FC := fun (R S : CommRingCat.{u}) ↦
                                            (↑R : Type u) →+* (↑S : Type u)) (CC := CommRingCat.carrier) RingCat (FD :=
                                            fun (R S : RingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CD :=
                                            RingCat.carrier))
                                        x) :
                                    Type u)))
                              (𝒞 :=
                              ModuleCat.moduleCategory
                                (↑(Functor.obj
                                      (R ⋙
                                        forget₂ CommRingCat (FC := fun (R S : CommRingCat.{u}) ↦
                                          (↑R : Type u) →+* (↑S : Type u)) (CC := CommRingCat.carrier) RingCat (FD :=
                                          fun (R S : RingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CD :=
                                          RingCat.carrier))
                                      x) :
                                  Type u)))
                            (M.obj x)))
                    Y))) :
          (a : (↑(M.obj X) : Type u)) →
            (↑((ModuleCat.restrictScalars (RingCat.Hom.hom ((R ⋙ forget₂ CommRingCat RingCat).map f))).obj
                  ((Monoidal.tensorObj (unit (R ⋙ forget₂ CommRingCat RingCat)) M).obj Y)) :
              Type u))
        m :
      (↑(Functor.obj
            (ModuleCat.restrictScalars
              (RingCat.Hom.hom
                (CategoryTheory.Functor.map
                  (R ⋙
                    forget₂ CommRingCat (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
                      CommRingCat.carrier) RingCat (FD := fun (R S : RingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u))
                      (CD := RingCat.carrier))
                  f)))
            ((Monoidal.tensorObj
                  (unit
                    (R ⋙
                      forget₂ CommRingCat (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
                        CommRingCat.carrier) RingCat (FD := fun (R S : RingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u))
                        (CD := RingCat.carrier)))
                  M).obj
              Y)) :
        Type u))
    ((ModuleCat.Hom.hom
            (Iso.hom
                ((fun (x : Cᵒᵖ) ↦
                    (Iso.symm (λ_ (M.obj x)) :
                      M.obj x ≅
                        MonoidalCategoryStruct.tensorObj (𝒞 :=
                          ModuleCat.moduleCategory
                            (↑(Functor.obj
                                  (R ⋙
                                    forget₂ CommRingCat (FC := fun (R S : CommRingCat.{u}) ↦
                                      (↑R : Type u) →+* (↑S : Type u)) (CC := CommRingCat.carrier) RingCat (FD :=
                                      fun (R S : RingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CD :=
                                      RingCat.carrier))
                                  x) :
                              Type u))
                          ((𝟙_
                              (ModuleCat
                                (↑(Functor.obj
                                      (R ⋙
                                        forget₂ CommRingCat (FC := fun (R S : CommRingCat.{u}) ↦
                                          (↑R : Type u) →+* (↑S : Type u)) (CC := CommRingCat.carrier) RingCat (FD :=
                                          fun (R S : RingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CD :=
                                          RingCat.carrier))
                                      x) :
                                  Type u)))
                            (𝒞 :=
                            ModuleCat.moduleCategory
                              (↑(Functor.obj
                                    (R ⋙
                                      forget₂ CommRingCat (FC := fun (R S : CommRingCat.{u}) ↦
                                        (↑R : Type u) →+* (↑S : Type u)) (CC := CommRingCat.carrier) RingCat (FD :=
                                        fun (R S : RingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CD :=
                                        RingCat.carrier))
                                    x) :
                                Type u)))
                          (M.obj x)))
                  X) ≫
              (Monoidal.tensorObj
                    (unit
                      (R ⋙
                        forget₂ CommRingCat (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
                          CommRingCat.carrier) RingCat (FD := fun (R S : RingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u))
                          (CD := RingCat.carrier)))
                    M).map
                f) :
          (a : (↑(M.obj X) : Type u)) →
            (↑((ModuleCat.restrictScalars (RingCat.Hom.hom ((R ⋙ forget₂ CommRingCat RingCat).map f))).obj
                  ((Monoidal.tensorObj (unit (R ⋙ forget₂ CommRingCat RingCat)) M).obj Y)) :
              Type u))
        m :
      (↑(Functor.obj
            (ModuleCat.restrictScalars
              (RingCat.Hom.hom
                (CategoryTheory.Functor.map
                  (R ⋙
                    forget₂ CommRingCat (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
                      CommRingCat.carrier) RingCat (FD := fun (R S : RingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u))
                      (CD := RingCat.carrier))
                  f)))
            ((Monoidal.tensorObj
                  (unit
                    (R ⋙
                      forget₂ CommRingCat (FC := fun (R S : CommRingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u)) (CC :=
                        CommRingCat.carrier) RingCat (FD := fun (R S : RingCat.{u}) ↦ (↑R : Type u) →+* (↑S : Type u))
                        (CD := RingCat.carrier)))
                  M).obj
              Y)) :
        Type u))
Tactic: ext m