-- 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_1 : K
k : (J.Cover (unop X))ᵒᵖ
⊢ Eq (α :=
    Functor.obj (J.diagram (limit.cone F).pt (unop X)) k ⟶
      (F ⋙ J.plusFunctor D ⋙ Functor.obj (evaluation Cᵒᵖ D) X).obj k_1)
    (CategoryStruct.comp (Y := colimit (J := (J.Cover (unop X))ᵒᵖ) (J.diagram (limit.cone F).pt (unop X)))
        (colimit.ι (J.diagram (limit.cone F).pt (unop X)) k)
        (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)
                          ⋯))))
                  (limit.π (F ⋙ J.plusFunctor D ⋙ Functor.obj (evaluation Cᵒᵖ D) X) k_1) :
                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_1) :
            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_1)) :
      Functor.obj (J.diagram (limit.cone F).pt (unop X)) k ⟶
        (F ⋙ J.plusFunctor D ⋙ Functor.obj (evaluation Cᵒᵖ D) X).obj k_1)
    (CategoryStruct.comp (Y := colimit (J := (J.Cover (unop X))ᵒᵖ) (J.diagram (limit.cone F).pt (unop X)))
        (colimit.ι (J.diagram (limit.cone F).pt (unop X)) k)
        (NatTrans.app (Cone.π (Functor.mapCone (J.plusFunctor D ⋙ Functor.obj (evaluation Cᵒᵖ D) X) (limit.cone F)))
          k_1) :
      Functor.obj (J.diagram (limit.cone F).pt (unop X)) k ⟶
        (F ⋙ J.plusFunctor D ⋙ Functor.obj (evaluation Cᵒᵖ D) X).obj k_1)
have preservesLimit : Eq (α :=
  multiequalizer ((unop k).index (limit F)) ⟶ colimit (J := (J.Cover (unop X))ᵒᵖ) (J.diagram (F.obj k_1) (unop X)))
  (CategoryStruct.comp (Y := colimit (J := (J.Cover (unop X))ᵒᵖ) (J.diagram (limit F) (unop X)))
      (colimit.ι (J.diagram (limit F) (unop X)) k :
        Functor.obj (J.diagram (limit F) (unop X)) k ⟶
          colimit (J := (J.Cover (unop X))ᵒᵖ) (J.diagram (limit F) (unop X)))
      (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))))) :
            colimit (J := (J.Cover (unop X))ᵒᵖ) (J.diagram (limit F) (unop X)) ⟶
              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_1) :
                limit (colimit (J := (J.Cover (unop X))ᵒᵖ) (Functor.flip (F ⋙ J.diagramFunctor D (unop X)))) ⟶
                  colimit (J := (J.Cover (unop X))ᵒᵖ) (J.diagram (F.obj k_1) (unop X))) :
            colimit (J := (J.Cover (unop X))ᵒᵖ) (limit (F ⋙ J.diagramFunctor D (unop X))) ⟶
              colimit (J := (J.Cover (unop X))ᵒᵖ) (J.diagram (F.obj k_1) (unop X))) :
        colimit (J := (J.Cover (unop X))ᵒᵖ) (J.diagram (limit F) (unop X)) ⟶
          colimit (J := (J.Cover (unop X))ᵒᵖ) (J.diagram (F.obj k_1) (unop X))) :
    multiequalizer ((unop k).index (limit F)) ⟶ colimit (J := (J.Cover (unop X))ᵒᵖ) (J.diagram (F.obj k_1) (unop X)))
  (CategoryStruct.comp (Y := colimit (J := (J.Cover (unop X))ᵒᵖ) (J.diagram (limit F) (unop X)))
      (colimit.ι (J.diagram (limit F) (unop X)) k) (colimMap (J.diagramNatTrans (limit.π F k_1) (unop X))) :
    multiequalizer ((unop k).index (limit F)) ⟶ colimit (J := (J.Cover (unop X))ᵒᵖ) (J.diagram (F.obj k_1) (unop X)))
Tactic: dsimp [plusMap, plusObj]