-- Lean 4 proof step from Mathlib.Topology.Sheaves.EtaleSpace (goal)
Goal:
X : TopCat
C : Type u
inst : Category.{v, u} C
CC : C → Type v
FC : C → C → Type w
inst_1 : (X Y : C) → FunLike.{w + 1, v + 1, v + 1} (FC X Y) (CC X) (CC Y)
inst_2 : ConcreteCategory C FC (CC := CC)
inst_3 : Limits.HasColimits.{v, u} C
F : Presheaf C X
inst_4 : Limits.PreservesFilteredColimits (forget C (FC := FC) (CC := CC))
U : Opens (↑X : Type v)
hF_bij :
  ∀ (x : (↑X : Type v)) (hx : x ∈ U),
    Bijective.{v + 1, v + 1} (α := CC (F.obj (op U))) (β := CC (F.stalk x))
      (⇑(ConcreteCategory.hom (FC := FC) (CC := CC) (F.germ U x hx)) : (a : CC (F.obj (op U))) → CC (F.stalk x))
x : (↑X : Type v)
hx : x ∈ U
g : F.EtaleSpace (CC := CC) (FC := FC)
hg :
  Membership.mem (γ := Set (F.EtaleSpace (CC := CC) (FC := FC)))
    (base (CC := CC) (FC := FC) ⁻¹' (↑U : Set (↑X : Type v))) g
f : CC (F.obj (op U))
hf :
  (ConcreteCategory.hom (FC := FC) (CC := CC) (F.germ U (g.base (CC := CC) (FC := FC)) hg) :
        (a : CC (F.obj (op U))) → CC (F.stalk g.base))
      f =
    g.germ (CC := CC) (FC := FC)
⊢ ∀ᶠ (b : F.EtaleSpace (CC := CC) (FC := FC)) in
    𝓝
      (↑(Subtype.mk (p := fun (x : F.EtaleSpace (CC := CC) (FC := FC)) ↦
            Membership.mem (γ := Set (F.EtaleSpace (CC := CC) (FC := FC)))
              (base (CC := CC) (FC := FC) ⁻¹' (↑U : Set (↑X : Type v))) x)
            g hg) :
        F.EtaleSpace),
    ∀ (a : (↑(base (CC := CC) (FC := FC) ⁻¹' (↑U : Set (↑X : Type v))) : Type v)),
      (↑a : F.EtaleSpace) = b →
        Eq (α := WithDiscreteTopology (ToType (FC := FC) (CC := CC) (F.stalk x)))
          (WithTopology.toTopology ⊥
              ((ConcreteCategory.hom (FC := FC) (CC := CC) (F.germ U x hx) : (a : CC (F.obj (op U))) → CC (F.stalk x))
                (surjInv ⋯ ((↑a : F.EtaleSpace).germ (CC := CC) (FC := FC)))) :
            WithTopology (ToType (FC := FC) (CC := CC) (F.stalk x)) ⊥)
          (WithTopology.toTopology ⊥
              ((ConcreteCategory.hom (FC := FC) (CC := CC) (F.germ U x hx) : (a : CC (F.obj (op U))) → CC (F.stalk x))
                (surjInv ⋯
                  ((↑(Subtype.mk (p := fun (x : F.EtaleSpace (CC := CC) (FC := FC)) ↦
                            Membership.mem (γ := Set (F.EtaleSpace (CC := CC) (FC := FC)))
                              (base (CC := CC) (FC := FC) ⁻¹' (↑U : Set (↑X : Type v))) x)
                            g hg) :
                        F.EtaleSpace).germ
                    (CC := CC) (FC := FC)))) :
            WithTopology (ToType (FC := FC) (CC := CC) (F.stalk x)) ⊥)
have h : ∀ (a : F.EtaleSpace (CC := CC) (FC := FC)),
  (∃ (hgU : a.base (CC := CC) (FC := FC) ∈ U),
      a.germ (CC := CC) (FC := FC) =
        (ConcreteCategory.hom (FC := FC) (CC := CC) (F.germ U (a.base (CC := CC) (FC := FC)) hgU) :
            (a_1 : CC (F.obj (op U))) → CC (F.stalk a.base))
          f) →
    ∀ (a_2 : (↑(base (CC := CC) (FC := FC) ⁻¹' (↑U : Set (↑X : Type v))) : Type v)),
      (↑a_2 : F.EtaleSpace) = a →
        Eq (α := WithDiscreteTopology (ToType (FC := FC) (CC := CC) (F.stalk x)))
          (WithTopology.toTopology ⊥
              ((ConcreteCategory.hom (FC := FC) (CC := CC) (F.germ U x hx) : (a : CC (F.obj (op U))) → CC (F.stalk x))
                (surjInv sorry ((↑a_2 : F.EtaleSpace).germ (CC := CC) (FC := FC)))) :
            WithTopology (ToType (FC := FC) (CC := CC) (F.stalk x)) ⊥)
          (WithTopology.toTopology ⊥
              ((ConcreteCategory.hom (FC := FC) (CC := CC) (F.germ U x hx) : (a : CC (F.obj (op U))) → CC (F.stalk x))
                (surjInv sorry (g.germ (CC := CC) (FC := FC)))) :
            WithTopology (ToType (FC := FC) (CC := CC) (F.stalk x)) ⊥)
Tactic: filter_upwards [g.eventually_nhds hg f hf]