-- Lean 4 proof step from Mathlib.Algebra.Category.ModuleCat.Presheaf.Limits (goal)
Goal:
C : Type u₁
inst : Category.{v₁, u₁} C
R : CategoryTheory.Functor.{v₁, u, u₁, u + 1} Cᵒᵖ RingCat
J : Type u₂
inst_1 : Category.{v₂, u₂} J
F : J ⥤ PresheafOfModules.{v, v₁, u₁, u} R
inst_2 :
  ∀ (X : Cᵒᵖ),
    Small.{v, max u₂ v}
      (↑((F ⋙ evaluation R X) ⋙
              forget (ModuleCat.{v, u} (↑(R.obj X) : Type u)) (FC :=
                fun (x1 x2 : ModuleCat.{v, u} (↑(R.obj X) : Type u)) ↦
                LinearMap.{u, u, v, v} (R := (↑(R.obj X) : Type u)) (RingHom.id (↑(R.obj X) : Type u)) (↑x1 : Type v)
                  (↑x2 : Type v))
                (CC := ModuleCat.carrier)).sections :
        Type (max u₂ v))
X Y Z : Cᵒᵖ
f : X ⟶ Y
g : Y ⟶ Z
⊢ Eq (α :=
    limit (F ⋙ evaluation R X) ⟶
      Functor.obj (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map (f ≫ g)))) (limit (F ⋙ evaluation R Z)))
    (CategoryStruct.comp (Y := limit (F ⋙ evaluation R Z ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map (f ≫ g)))))
      (limMap (F.whiskerLeft (restriction R (f ≫ g))))
      (Iso.inv (preservesLimitIso (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map (f ≫ g)))) (F ⋙ evaluation R Z))))
    (CategoryStruct.comp (Y := (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map f))).obj (limit (F ⋙ evaluation R Y)))
      (CategoryStruct.comp (Y := limit (F ⋙ evaluation R Y ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map f))))
        (limMap (F.whiskerLeft (restriction R f)) :
          limit (F ⋙ evaluation R X) ⟶
            limit (F ⋙ evaluation R Y ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map f))))
        (Iso.inv (preservesLimitIso (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map f))) (F ⋙ evaluation R Y)) :
          limit ((F ⋙ evaluation R Y) ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map f))) ⟶
            (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map f))).obj (limit (F ⋙ evaluation R Y))))
      ((ModuleCat.restrictScalars (RingCat.Hom.hom (R.map f))).map
          (CategoryStruct.comp (Y := limit (F ⋙ evaluation R Z ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map g))))
            (limMap (F.whiskerLeft (restriction R g)) :
              limit (F ⋙ evaluation R Y) ⟶
                limit (F ⋙ evaluation R Z ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map g))))
            (Iso.inv (preservesLimitIso (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map g))) (F ⋙ evaluation R Z)) :
              limit ((F ⋙ evaluation R Z) ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map g))) ⟶
                (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map g))).obj (limit (F ⋙ evaluation R Z)))) ≫
        Iso.inv
          (ModuleCat.restrictScalarsComp'App (RingCat.Hom.hom (R.map f)) (RingCat.Hom.hom (R.map g))
            (RingCat.Hom.hom (R.map (f ≫ g))) ⋯ (limit (F ⋙ evaluation R Z)))))
have rw : Eq (α :=
  limit (F ⋙ evaluation R X) ⟶
    limit ((F ⋙ evaluation R Z) ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map (f ≫ g)))))
  (limMap (F.whiskerLeft (restriction R (f ≫ g))) :
    limit (F ⋙ evaluation R X) ⟶
      limit (F ⋙ evaluation R Z ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map (f ≫ g)))))
  (CategoryStruct.comp (Y := limit (F ⋙ evaluation R Y ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map f))))
      (limMap (F.whiskerLeft (restriction R f)) :
        limit (F ⋙ evaluation R X) ⟶ limit (F ⋙ evaluation R Y ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map f))))
      (CategoryStruct.comp (Y :=
          (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map f))).obj (limit (F ⋙ evaluation R Y)))
          (Iso.inv (preservesLimitIso (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map f))) (F ⋙ evaluation R Y)))
          (CategoryStruct.comp (Y :=
            (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map f))).obj
              ((ModuleCat.restrictScalars (RingCat.Hom.hom (R.map g))).obj (limit (F ⋙ evaluation R Z))))
            ((ModuleCat.restrictScalars (RingCat.Hom.hom (R.map f))).map
              (CategoryStruct.comp (Y :=
                limit (F ⋙ evaluation R Z ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map g))))
                (limMap (F.whiskerLeft (restriction R g)) :
                  limit (F ⋙ evaluation R Y) ⟶
                    limit (F ⋙ evaluation R Z ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map g))))
                (Iso.inv
                    (preservesLimitIso (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map g))) (F ⋙ evaluation R Z)) :
                  limit ((F ⋙ evaluation R Z) ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map g))) ⟶
                    (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map g))).obj (limit (F ⋙ evaluation R Z)))))
            (Iso.inv
                (ModuleCat.restrictScalarsComp'App (RingCat.Hom.hom (R.map f)) (RingCat.Hom.hom (R.map g))
                  (RingCat.Hom.hom (R.map (f ≫ g))) sorry (limit (F ⋙ evaluation R Z))) ≫
              Iso.hom
                (preservesLimitIso (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map (f ≫ g))))
                  (F ⋙ evaluation R Z)))) :
        limit (F ⋙ evaluation R Y ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map f))) ⟶
          limit ((F ⋙ evaluation R Z) ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map (f ≫ g))))) :
    limit (F ⋙ evaluation R X) ⟶
      limit ((F ⋙ evaluation R Z) ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map (f ≫ g)))))
Tactic: rw [← cancel_mono (preservesLimitIso _ _).hom, assoc, assoc, assoc, assoc, Iso.inv_hom_id,
      comp_id]