-- 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 : D.J
U : Opens (↑(↑(D.U i) : TopCat) : Type v)
rw :
  Eq (α :=
    limit (D.diagramOverOpen U) ⟶
      (componentwiseDiagram D.diagram.multispan
            (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))).obj
        (op (WalkingMultispan.left (i, j))))
    (CategoryStruct.comp (Y :=
      (componentwiseDiagram D.diagram.multispan
            (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))).obj
        (op (WalkingMultispan.right ((MultispanShape.prod D.J).snd (i, j)))))
      (D.diagramOverOpenπ U i ≫ D.ιInvAppπEqMap U ≫ D.ιInvApp U ≫ D.diagramOverOpenπ U j :
        limit (D.diagramOverOpen U) ⟶ (D.diagramOverOpen U).obj (op (WalkingMultispan.right j)))
      ((componentwiseDiagram D.diagram.multispan
              (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))).map
          (Quiver.Hom.op (WalkingMultispan.Hom.snd (i, j))) ≫
        𝟙
          ((componentwiseDiagram D.diagram.multispan
                (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))).obj
            (op (WalkingMultispan.left (i, j))))))
    (D.diagramOverOpenπ U j ≫
      (componentwiseDiagram D.diagram.multispan
              (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))).map
          (Quiver.Hom.op (WalkingMultispan.Hom.snd (i, j))) ≫
        𝟙
          ((componentwiseDiagram D.diagram.multispan
                (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))).obj
            (op (WalkingMultispan.left (i, j)))))
⊢ Eq (α := limit (D.diagramOverOpen U) ⟶ (D.diagramOverOpen U).obj (op (WalkingMultispan.right j)))
    (D.diagramOverOpenπ U i ≫ D.ιInvAppπEqMap U ≫ D.ιInvApp U ≫ D.diagramOverOpenπ U j) (D.diagramOverOpenπ U j)
have rw₁ : Mono (X :=
  (componentwiseDiagram D.diagram.multispan
        (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))).obj
    (op (WalkingMultispan.right ((MultispanShape.prod D.J).snd (i, j)))))
  ((componentwiseDiagram D.diagram.multispan
          (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))).map
      (Quiver.Hom.op (WalkingMultispan.Hom.snd (i, j))) ≫
    𝟙
      ((componentwiseDiagram D.diagram.multispan
            (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))).obj
        (op (WalkingMultispan.left (i, j)))))
Tactic: rw [← @cancel_mono
          (f := (componentwiseDiagram 𝖣.diagram.multispan _).map
            (Quiver.Hom.op (WalkingMultispan.Hom.snd (i, j))) ≫ 𝟙 _) ..]