-- Lean 4 proof step from Mathlib.Geometry.RingedSpace.PresheafedSpace.Gluing (goal)
Goal:
C : Type u
inst : Category.{v, u} C
D : GlueData.{v, u} C
inst_1 : HasLimits.{v, u} C
i j k : D.J
U : Opens (↑(↑(D.U i) : TopCat) : Type v)
h :
  Eq (α :=
    (D.U i).presheaf.obj (op U) ⟶
      (Functor.obj (TopCat.Presheaf.pushforward C (D.f j k).base) (D.V (j, k)).presheaf).obj
        (op
          (Functor.obj (C := Opens (↑(↑D.glued : TopCat) : Type v))
            (Opens.map (Hom.base (D.ι j)) :
              Opens (↑(↑D.glued : TopCat) : Type v) ⥤ Opens (↑(↑(D.U j) : TopCat) : Type v))
            (Functor.obj
                (IsOpenEmbedding.functor (f := Hom.base (D.ι i)) ⋯ :
                  Opens (↑(↑(D.U i) : TopCat) : Type v) ⥤ Opens (↑(↑D.glued : TopCat) : Type v))
                U :
              Opens (↑(↑D.glued : TopCat) : Type v)))))
    (CategoryStruct.comp (Y :=
        (D.U j).presheaf.obj
          (op
            (Functor.obj (C := Opens (↑(↑D.glued : TopCat) : Type v))
              (Opens.map (Hom.base (D.ι j)) :
                Opens (↑(↑D.glued : TopCat) : Type v) ⥤ Opens (↑(↑(D.U j) : TopCat) : Type v))
              (Functor.obj
                  (IsOpenEmbedding.functor (f := Hom.base (D.ι i)) ⋯ :
                    Opens (↑(↑(D.U i) : TopCat) : Type v) ⥤ Opens (↑(↑D.glued : TopCat) : Type v))
                  U :
                Opens (↑(↑D.glued : TopCat) : Type v)))))
        (D.opensImagePreimageMap i j U :
          (D.U i).presheaf.obj (op U) ⟶
            (D.U j).presheaf.obj
              (op
                (Functor.obj (C := Opens (↑(↑D.glued : TopCat) : Type v))
                  (Opens.map (Hom.base (D.ι j)) :
                    Opens (↑(↑D.glued : TopCat) : Type v) ⥤ Opens (↑(↑(D.U j) : TopCat) : Type v))
                  (Functor.obj
                      (IsOpenEmbedding.functor (f := Hom.base (D.ι i)) ⋯ :
                        Opens (↑(↑(D.U i) : TopCat) : Type v) ⥤ Opens (↑(↑D.glued : TopCat) : Type v))
                      U :
                    Opens (↑(↑D.glued : TopCat) : Type v)))))
        ((D.f j k).c.app
            (op
              (Functor.obj (C := Opens (↑(↑D.glued : TopCat) : Type v))
                (Opens.map (Hom.base (D.ι j)) :
                  Opens (↑(↑D.glued : TopCat) : Type v) ⥤ Opens (↑(↑(D.U j) : TopCat) : Type v))
                (Functor.obj
                    (IsOpenEmbedding.functor (f := Hom.base (D.ι i)) ⋯ :
                      Opens (↑(↑(D.U i) : TopCat) : Type v) ⥤ Opens (↑(↑D.glued : TopCat) : Type v))
                    U :
                  Opens (↑(↑D.glued : TopCat) : Type v)))) :
          (D.U j).presheaf.obj
              (op
                (Functor.obj (C := Opens (↑(↑D.glued : TopCat) : Type v))
                  (Opens.map (Hom.base (D.ι j)) :
                    Opens (↑(↑D.glued : TopCat) : Type v) ⥤ Opens (↑(↑(D.U j) : TopCat) : Type v))
                  (Functor.obj
                      (IsOpenEmbedding.functor (f := Hom.base (D.ι i)) ⋯ :
                        Opens (↑(↑(D.U i) : TopCat) : Type v) ⥤ Opens (↑(↑D.glued : TopCat) : Type v))
                      U :
                    Opens (↑(↑D.glued : TopCat) : Type v)))) ⟶
            (Functor.obj (TopCat.Presheaf.pushforward C (D.f j k).base) (D.V (j, k)).presheaf).obj
              (op
                (Functor.obj (C := Opens (↑(↑D.glued : TopCat) : Type v))
                  (Opens.map (Hom.base (D.ι j)) :
                    Opens (↑(↑D.glued : TopCat) : Type v) ⥤ Opens (↑(↑(D.U j) : TopCat) : Type v))
                  (Functor.obj
                      (IsOpenEmbedding.functor (f := Hom.base (D.ι i)) ⋯ :
                        Opens (↑(↑(D.U i) : TopCat) : Type v) ⥤ Opens (↑(↑D.glued : TopCat) : Type v))
                      U :
                    Opens (↑(↑D.glued : TopCat) : Type v))))) :
      (D.U i).presheaf.obj (op U) ⟶
        (Functor.obj (TopCat.Presheaf.pushforward C (D.f j k).base) (D.V (j, k)).presheaf).obj
          (op
            (Functor.obj (C := Opens (↑(↑D.glued : TopCat) : Type v))
              (Opens.map (Hom.base (D.ι j)) :
                Opens (↑(↑D.glued : TopCat) : Type v) ⥤ Opens (↑(↑(D.U j) : TopCat) : Type v))
              (Functor.obj
                  (IsOpenEmbedding.functor (f := Hom.base (D.ι i)) ⋯ :
                    Opens (↑(↑(D.U i) : TopCat) : Type v) ⥤ Opens (↑(↑D.glued : TopCat) : Type v))
                  U :
                Opens (↑(↑D.glued : TopCat) : Type v)))))
    (CategoryStruct.comp (Y :=
        (Functor.obj (TopCat.Presheaf.pushforward C (Hom.base (pullback.fst (D.f j i) (D.f j k) ≫ D.t j i ≫ D.f i j)))
              (pullback (D.f j i) (D.f j k)).presheaf).obj
          (op U))
        (NatTrans.app (Hom.c (pullback.fst (D.f j i) (D.f j k) ≫ D.t j i ≫ D.f i j)) (op U))
        (invApp (pullback.snd (D.f j i) (D.f j k))
            (unop
              (op
                ((Opens.map (Hom.base (pullback.fst (D.f j i) (D.f j k) ≫ D.t j i ≫ D.f i j)) :
                      Opens (↑(↑(D.U i) : TopCat) : Type v) ⥤
                        Opens (↑(↑(pullback (D.f j i) (D.f j k)) : TopCat) : Type v)).1
                  (unop (op U))))) ≫
          (D.V (j, k)).presheaf.map (eqToHom ⋯)) :
      (D.U i).presheaf.obj (op U) ⟶
        (D.V (j, k)).presheaf.obj
          ((Opens.map (D.f j k).base).op.obj
            (op
              (Functor.obj (C := Opens (↑(↑D.glued : TopCat) : Type v))
                (Opens.map (Hom.base (D.ι j)) :
                  Opens (↑(↑D.glued : TopCat) : Type v) ⥤ Opens (↑(↑(D.U j) : TopCat) : Type v))
                (Functor.obj
                    (IsOpenEmbedding.functor (f := Hom.base (D.ι i)) ⋯ :
                      Opens (↑(↑(D.U i) : TopCat) : Type v) ⥤ Opens (↑(↑D.glued : TopCat) : Type v))
                    U :
                  Opens (↑(↑D.glued : TopCat) : Type v))))))
⊢ ∃ (eq :
    op
        (Functor.obj (C := Opens (↑(↑(pullback (D.f j i) (D.f j k)) : TopCat) : Type v))
          (opensFunctor (pullback.snd (D.f j i) (D.f j k)))
          (unop
            (op
              ((Opens.map (Hom.base (pullback.fst (D.f j i) (D.f j k) ≫ D.t j i ≫ D.f i j)) :
                    Opens (↑(↑(D.U i) : TopCat) : Type v) ⥤
                      Opens (↑(↑(pullback (D.f j i) (D.f j k)) : TopCat) : Type v)).1
                (unop (op U)))))) =
      (Opens.map (D.f j k).base).op.obj
        (op
          (Functor.obj (C := Opens (↑(↑D.glued : TopCat) : Type v))
            (Opens.map (Hom.base (D.ι j)) :
              Opens (↑(↑D.glued : TopCat) : Type v) ⥤ Opens (↑(↑(D.U j) : TopCat) : Type v))
            (Functor.obj
                (IsOpenEmbedding.functor (f := Hom.base (D.ι i)) ⋯ :
                  Opens (↑(↑(D.U i) : TopCat) : Type v) ⥤ Opens (↑(↑D.glued : TopCat) : Type v))
                U :
              Opens (↑(↑D.glued : TopCat) : Type v))))),
    CategoryStruct.comp (Y :=
        (D.U j).presheaf.obj
          (op
            (Functor.obj (C := Opens (↑(↑D.glued : TopCat) : Type v))
              (Opens.map (Hom.base (D.ι j)) :
                Opens (↑(↑D.glued : TopCat) : Type v) ⥤ Opens (↑(↑(D.U j) : TopCat) : Type v))
              (Functor.obj
                  (IsOpenEmbedding.functor (f := Hom.base (D.ι i)) ⋯ :
                    Opens (↑(↑(D.U i) : TopCat) : Type v) ⥤ Opens (↑(↑D.glued : TopCat) : Type v))
                  U :
                Opens (↑(↑D.glued : TopCat) : Type v)))))
        (D.opensImagePreimageMap i j U :
          (D.U i).presheaf.obj (op U) ⟶
            (D.U j).presheaf.obj
              (op
                (Functor.obj (C := Opens (↑(↑D.glued : TopCat) : Type v))
                  (Opens.map (Hom.base (D.ι j)) :
                    Opens (↑(↑D.glued : TopCat) : Type v) ⥤ Opens (↑(↑(D.U j) : TopCat) : Type v))
                  (Functor.obj
                      (IsOpenEmbedding.functor (f := Hom.base (D.ι i)) ⋯ :
                        Opens (↑(↑(D.U i) : TopCat) : Type v) ⥤ Opens (↑(↑D.glued : TopCat) : Type v))
                      U :
                    Opens (↑(↑D.glued : TopCat) : Type v)))))
        ((D.f j k).c.app
            (op
              (Functor.obj (C := Opens (↑(↑D.glued : TopCat) : Type v))
                (Opens.map (Hom.base (D.ι j)) :
                  Opens (↑(↑D.glued : TopCat) : Type v) ⥤ Opens (↑(↑(D.U j) : TopCat) : Type v))
                (Functor.obj
                    (IsOpenEmbedding.functor (f := Hom.base (D.ι i)) ⋯ :
                      Opens (↑(↑(D.U i) : TopCat) : Type v) ⥤ Opens (↑(↑D.glued : TopCat) : Type v))
                    U :
                  Opens (↑(↑D.glued : TopCat) : Type v)))) :
          (D.U j).presheaf.obj
              (op
                (Functor.obj (C := Opens (↑(↑D.glued : TopCat) : Type v))
                  (Opens.map (Hom.base (D.ι j)) :
                    Opens (↑(↑D.glued : TopCat) : Type v) ⥤ Opens (↑(↑(D.U j) : TopCat) : Type v))
                  (Functor.obj
                      (IsOpenEmbedding.functor (f := Hom.base (D.ι i)) ⋯ :
                        Opens (↑(↑(D.U i) : TopCat) : Type v) ⥤ Opens (↑(↑D.glued : TopCat) : Type v))
                      U :
                    Opens (↑(↑D.glued : TopCat) : Type v)))) ⟶
            (Functor.obj (TopCat.Presheaf.pushforward C (D.f j k).base) (D.V (j, k)).presheaf).obj
              (op
                (Functor.obj (C := Opens (↑(↑D.glued : TopCat) : Type v))
                  (Opens.map (Hom.base (D.ι j)) :
                    Opens (↑(↑D.glued : TopCat) : Type v) ⥤ Opens (↑(↑(D.U j) : TopCat) : Type v))
                  (Functor.obj
                      (IsOpenEmbedding.functor (f := Hom.base (D.ι i)) ⋯ :
                        Opens (↑(↑(D.U i) : TopCat) : Type v) ⥤ Opens (↑(↑D.glued : TopCat) : Type v))
                      U :
                    Opens (↑(↑D.glued : TopCat) : Type v))))) =
      NatTrans.app (Hom.c (pullback.fst (D.f j i) (D.f j k) ≫ D.t j i ≫ D.f i j)) (op U) ≫
        invApp (pullback.snd (D.f j i) (D.f j k))
            (unop
              (op
                ((Opens.map (Hom.base (pullback.fst (D.f j i) (D.f j k) ≫ D.t j i ≫ D.f i j)) :
                      Opens (↑(↑(D.U i) : TopCat) : Type v) ⥤
                        Opens (↑(↑(pullback (D.f j i) (D.f j k)) : TopCat) : Type v)).1
                  (unop (op U))))) ≫
          (D.V (j, k)).presheaf.map (eqToHom eq)
have w : op
    (Functor.obj (C := Opens (↑(↑(pullback (D.f j i) (D.f j k)) : TopCat) : Type v))
      (opensFunctor (pullback.snd (D.f j i) (D.f j k)))
      (unop
        (op
          ((Opens.map (Hom.base (pullback.fst (D.f j i) (D.f j k) ≫ D.t j i ≫ D.f i j)) :
                Opens (↑(↑(D.U i) : TopCat) : Type v) ⥤ Opens (↑(↑(pullback (D.f j i) (D.f j k)) : TopCat) : Type v)).1
            (unop (op U)))))) =
  (Opens.map (D.f j k).base).op.obj
    (op
      (Functor.obj (C := Opens (↑(↑D.glued : TopCat) : Type v))
        (Opens.map (Hom.base (D.ι j)) : Opens (↑(↑D.glued : TopCat) : Type v) ⥤ Opens (↑(↑(D.U j) : TopCat) : Type v))
        (Functor.obj
            (IsOpenEmbedding.functor (f := Hom.base (D.ι i)) sorry :
              Opens (↑(↑(D.U i) : TopCat) : Type v) ⥤ Opens (↑(↑D.glued : TopCat) : Type v))
            U :
          Opens (↑(↑D.glued : TopCat) : Type v))))
Tactic: constructor