-- Lean 4 proof step from Mathlib.CategoryTheory.Sites.LeftExact (goal)
Goal:
C : Type u
inst : Category.{v, u} C
J : GrothendieckTopology.{v, u} C
D : Type w
inst_1 : Category.{t, w} D
inst_2 : ∀ (P : Functor.{v, t, u, w} Cᵒᵖ D) (X : C) (S : J.Cover X), HasMultiequalizer (S.index P)
inst_3 : ∀ (X : C), HasColimitsOfShape (J.Cover X)ᵒᵖ D
FD : D → D → Type u_1
CD : D → Type t
inst_4 : (X Y : D) → FunLike.{u_1 + 1, t + 1, t + 1} (FD X Y) (CD X) (CD Y)
inst_5 : ConcreteCategory D FD (CC := CD)
inst_6 : ∀ (X : C), PreservesColimitsOfShape (J.Cover X)ᵒᵖ (forget D (FC := FD) (CC := CD))
inst_7 : ∀ (X : C), Small.{t, max u v} (J.Cover X)ᵒᵖ
K : Type t
inst_8 : SmallCategory K
inst_9 : FinCategory K
inst_10 : HasLimitsOfShape K D
inst_11 : PreservesLimitsOfShape K (forget D (FC := FD) (CC := CD))
inst_12 : ReflectsLimitsOfShape K (forget D (FC := FD) (CC := CD))
F : K ⥤ Functor.{v, t, u, w} Cᵒᵖ D
X : Cᵒᵖ
S : Cone (F ⋙ J.plusFunctor D ⋙ Functor.obj (evaluation Cᵒᵖ D) X)
m :
  S.pt ⟶
    Cone.pt (F := F ⋙ J.plusFunctor D ⋙ Functor.obj (evaluation Cᵒᵖ D) X)
      (Functor.mapCone (J.plusFunctor D ⋙ Functor.obj (evaluation Cᵒᵖ D) X) (limit.cone F))
hm :
  ∀ (j : K),
    m ≫ NatTrans.app (Cone.π (Functor.mapCone (J.plusFunctor D ⋙ Functor.obj (evaluation Cᵒᵖ D) X) (limit.cone F))) j =
      S.π.app j
k : K
⊢ CategoryStruct.comp (Y := limit (F ⋙ J.plusFunctor D ⋙ Functor.obj (evaluation Cᵒᵖ D) X))
      (CategoryStruct.comp (Y :=
        limit (colimit (J := (J.Cover (unop X))ᵒᵖ) (Functor.flip (F ⋙ J.diagramFunctor D (unop X)))))
        (CategoryStruct.comp (Y := colimit (J := (J.Cover (unop X))ᵒᵖ) (limit (F ⋙ J.diagramFunctor D (unop X))))
            (m ≫
                Iso.hom
                  (HasColimit.isoOfNatIso
                    (IsLimit.conePointUniqueUpToIso (isLimitOfPreserves (J.diagramFunctor D (unop X)) (limit.isLimit F))
                      (limit.isLimit (F ⋙ J.diagramFunctor D (unop X))))) :
              S.pt ⟶ colimit (J := (J.Cover (unop X))ᵒᵖ) (limit (F ⋙ J.diagramFunctor D (unop X))))
            (Iso.hom (colimitLimitIso (F ⋙ J.diagramFunctor D (unop X))) :
              colimit (J := (J.Cover (unop X))ᵒᵖ) (limit (F ⋙ J.diagramFunctor D (unop X))) ⟶
                limit (colimit (J := (J.Cover (unop X))ᵒᵖ) (Functor.flip (F ⋙ J.diagramFunctor D (unop X))))) :
          S.pt ⟶ limit (colimit (J := (J.Cover (unop X))ᵒᵖ) (Functor.flip (F ⋙ J.diagramFunctor D (unop X)))))
        (Iso.inv
            (HasLimit.isoOfNatIso
              (Iso.symm
                (NatIso.ofComponents
                  (fun (k : K) ↦ colimitObjIsoColimitCompEvaluation (Functor.flip (F ⋙ J.diagramFunctor D (unop X))) k)
                  ⋯))) :
          limit (colimit (J := (J.Cover (unop X))ᵒᵖ) (Functor.flip (F ⋙ J.diagramFunctor D (unop X)))) ⟶
            limit (F ⋙ J.plusFunctor D ⋙ Functor.obj (evaluation Cᵒᵖ D) X)))
      (limit.π (F ⋙ J.plusFunctor D ⋙ Functor.obj (evaluation Cᵒᵖ D) X) k) =
    CategoryStruct.comp (Y := limit (F ⋙ J.plusFunctor D ⋙ Functor.obj (evaluation Cᵒᵖ D) X))
      (limit.lift (F ⋙ J.plusFunctor D ⋙ Functor.obj (evaluation Cᵒᵖ D) X) S)
      (limit.π (F ⋙ J.plusFunctor D ⋙ Functor.obj (evaluation Cᵒᵖ D) X) k)
have preservesLimit : Eq (α := S.pt ⟶ (F ⋙ J.plusFunctor D ⋙ Functor.obj (evaluation Cᵒᵖ D) X).obj k)
  (m ≫
    CategoryStruct.comp (Y := colimit (J := (J.Cover (unop X))ᵒᵖ) (limit (F ⋙ J.diagramFunctor D (unop X))))
      (Iso.hom
          (HasColimit.isoOfNatIso
            (IsLimit.conePointUniqueUpToIso (isLimitOfPreserves (J.diagramFunctor D (unop X)) (limit.isLimit F))
              (limit.isLimit (F ⋙ J.diagramFunctor D (unop X))))) :
        Cone.pt (F := F ⋙ J.plusFunctor D ⋙ Functor.obj (evaluation Cᵒᵖ D) X)
            (Functor.mapCone (J.plusFunctor D ⋙ Functor.obj (evaluation Cᵒᵖ D) X) (limit.cone F)) ⟶
          colimit (J := (J.Cover (unop X))ᵒᵖ) (limit (F ⋙ J.diagramFunctor D (unop X))))
      (CategoryStruct.comp (Y :=
          limit (colimit (J := (J.Cover (unop X))ᵒᵖ) (Functor.flip (F ⋙ J.diagramFunctor D (unop X)))))
          (Iso.hom (colimitLimitIso (F ⋙ J.diagramFunctor D (unop X))) :
            colimit (J := (J.Cover (unop X))ᵒᵖ) (limit (F ⋙ J.diagramFunctor D (unop X))) ⟶
              limit (colimit (J := (J.Cover (unop X))ᵒᵖ) (Functor.flip (F ⋙ J.diagramFunctor D (unop X)))))
          (CategoryStruct.comp (Y := limit (F ⋙ J.plusFunctor D ⋙ Functor.obj (evaluation Cᵒᵖ D) X))
              (Iso.inv
                (HasLimit.isoOfNatIso
                  (Iso.symm
                    (NatIso.ofComponents
                      (fun (k : K) ↦
                        colimitObjIsoColimitCompEvaluation (Functor.flip (F ⋙ J.diagramFunctor D (unop X))) k)
                      sorry))))
              (limit.π (F ⋙ J.plusFunctor D ⋙ Functor.obj (evaluation Cᵒᵖ D) X) k) :
            limit (colimit (J := (J.Cover (unop X))ᵒᵖ) (Functor.flip (F ⋙ J.diagramFunctor D (unop X)))) ⟶
              (F ⋙ J.plusFunctor D ⋙ Functor.obj (evaluation Cᵒᵖ D) X).obj k) :
        colimit (J := (J.Cover (unop X))ᵒᵖ) (limit (F ⋙ J.diagramFunctor D (unop X))) ⟶
          (F ⋙ J.plusFunctor D ⋙ Functor.obj (evaluation Cᵒᵖ D) X).obj k))
  (m ≫ NatTrans.app (Cone.π (Functor.mapCone (J.plusFunctor D ⋙ Functor.obj (evaluation Cᵒᵖ D) X) (limit.cone F))) k)
Tactic: simp only [limit.lift_π, Category.assoc, ← hm]