-- Lean 4 proof step from Mathlib.Topology.Sheaves.SheafCondition.PairwiseIntersections (goal)
Goal:
C : Type u_1
inst : Category.{v_1, u_1} C
X : TopCat
F : Sheaf.{w, v_1, u_1} C X
U V : Opens (↑X : Type w)
s_1 :
  PullbackCone (X := F.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑X : Type w))).obj (op U))
    (Y := F.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑X : Type w))).obj (op V))
    (F.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑X : Type w))).map
      (Quiver.Hom.op (homOfLE ⋯)))
    (F.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑X : Type w))).map
      (Quiver.Hom.op (homOfLE ⋯)))
ι : ULift.{w, 0} WalkingPair → Opens (↑X : Type w) :=
  fun (x : ULift.{w, 0} WalkingPair) ↦
    match (motive := ULift.{w, 0} WalkingPair → Opens (↑X : Type w)) x with
    | { down := j } => WalkingPair.casesOn (motive := fun (x : WalkingPair) ↦ Opens (↑X : Type w)) j U V
hι : U ⊔ V = iSup ι
s :
  PullbackCone (X := F.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑X : Type w))).obj (op U))
    (Y := F.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑X : Type w))).obj (op V))
    (F.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑X : Type w))).map
      (Quiver.Hom.op (homOfLE ⋯)))
    (F.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑X : Type w))).map
      (Quiver.Hom.op (homOfLE ⋯)))
m : s.pt ⟶ (F.interUnionPullbackCone U V).pt
h₁ : m ≫ (F.interUnionPullbackCone U V).fst = s.fst
h₂ : m ≫ (F.interUnionPullbackCone U V).snd = s.snd
a : ULift.{w, 0} WalkingPair
⊢ Eq (α :=
    s.pt ⟶
      F.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑X : Type w))).obj
        (op (ι { down := WalkingPair.left })))
    ((m ≫
          F.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑X : Type w))).map
            (Quiver.Hom.op (eqToHom ⋯))) ≫
        F.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑X : Type w))).map
          (Quiver.Hom.op (homOfLE ⋯)) :
      s.pt ⟶
        F.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑X : Type w))).obj
          (op (ι { down := WalkingPair.left })))
    ((F.interUnionPullbackConeLift U V s ≫
          F.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑X : Type w))).map
            (Quiver.Hom.op (eqToHom ⋯))) ≫
        F.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑X : Type w))).map
          (Quiver.Hom.op (homOfLE ⋯)) :
      s.pt ⟶
        F.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑X : Type w))).obj
          (op (ι { down := WalkingPair.left })))
have property : Eq (α :=
  s.pt ⟶
    F.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑X : Type w))).obj
      (op (ι { down := WalkingPair.left })))
  (m ≫
      F.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑X : Type w))).map
        (Quiver.Hom.op (eqToHom sorry) ≫ Quiver.Hom.op (homOfLE sorry)) :
    s.pt ⟶
      F.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑X : Type w))).obj
        (op (ι { down := WalkingPair.left })))
  (F.interUnionPullbackConeLift U V s ≫
      F.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑X : Type w))).map
        (Quiver.Hom.op (eqToHom sorry) ≫ Quiver.Hom.op (homOfLE sorry)) :
    s.pt ⟶
      F.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑X : Type w))).obj
        (op (ι { down := WalkingPair.left })))
Tactic: simp_rw [Category.assoc, ← F.1.map_comp]