-- Lean 4 proof step from Mathlib.Topology.Sheaves.Functors (goal)
Goal:
C : Type u
inst : Category.{v, u} C
X Y : TopCat
f_1 : X ⟶ Y
ι : Type w
U : ι → Opens (↑Y : Type w)
A : Type u_1
inst_1 : Category.{w, u_1} A
f : X ⟶ Y
hf :
  IsOpenEmbedding (X := (↑X : Type w)) (Y := (↑Y : Type w))
    (⇑(ConcreteCategory.hom (FC := fun (X Y : TopCat.{w}) ↦ C((↑X : Type w), (↑Y : Type w))) (CC := carrier) f) :
      (a : (↑X : Type w)) → (↑Y : Type w))
FA : A → A → Type u_2
CA : A → Type w
inst_2 : (X Y : A) → FunLike.{u_2 + 1, w + 1, w + 1} (FA X Y) (CA X) (CA Y)
inst_3 : ConcreteCategory A FA (CC := CA)
inst_4 : HasColimits.{w, u_1} A
inst_5 : HasLimits.{w, u_1} A
inst_6 : PreservesLimits (CategoryTheory.forget A (FC := FA) (CC := CA))
inst_7 : PreservesFilteredColimits (CategoryTheory.forget A (FC := FA) (CC := CA))
inst_8 : Functor.ReflectsIsomorphisms (CategoryTheory.forget A (FC := FA) (CC := CA))
X_1 Y_1 : TopCat.Sheaf A Y
u : X_1 ⟶ Y_1
refine_2 :
  Eq (α :=
    ObjectProperty.FullSubcategory.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑X : Type w)))
        (Functor.obj (presheafToSheaf (Opens.grothendieckTopology (↑X : Type w)) A)
          (Functor.obj (Presheaf.pullback A f)
            (X_1.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑Y : Type w)))))) ⟶
      hf.functor.op ⋙ Y_1.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑Y : Type w))))
    (sheafifyLift (Opens.grothendieckTopology (↑X : Type w))
        (NatTrans.app (Iso.hom ⋯.pullbackIso)
          (X_1.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑Y : Type w)))))
        ⋯ ≫
      ⋯.functor.op.whiskerLeft u.hom)
    (sheafifyLift (Opens.grothendieckTopology (↑X : Type w))
        (NatTrans.app (Iso.hom ⋯.pullbackIso)
          (X_1.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑Y : Type w)))))
        ⋯ ≫
      hf.functor.op.whiskerLeft u.hom)
⊢ Eq (α :=
    ObjectProperty.FullSubcategory.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑X : Type w)))
        (Functor.obj (presheafToSheaf (Opens.grothendieckTopology (↑X : Type w)) A)
          (Functor.obj (Presheaf.pullback A f)
            (X_1.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑Y : Type w)))))) ⟶
      hf.functor.op ⋙ Y_1.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑Y : Type w))))
    (sheafifyLift (Opens.grothendieckTopology (↑X : Type w))
      (NatTrans.app (Iso.hom ⋯.pullbackIso)
          (X_1.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑Y : Type w)))) ≫
        ⋯.functor.op.whiskerLeft u.hom)
      ⋯)
    (sheafifyLift (Opens.grothendieckTopology (↑X : Type w))
        (NatTrans.app (Iso.hom ⋯.pullbackIso)
          (X_1.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑Y : Type w)))))
        ⋯ ≫
      hf.functor.op.whiskerLeft u.hom)
have refine_2₁ : CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑X : Type w))
  (sorry.functor.op ⋙ X_1.obj (P := CategoryTheory.Presheaf.IsSheaf (Opens.grothendieckTopology (↑Y : Type w))))
Tactic: rw [CategoryTheory.sheafifyLift_comp]