-- Lean 4 proof step from Mathlib.CategoryTheory.Sites.Point.Conservative (goal)
Goal:
C : Type u
inst : Category.{v, u} C
J : GrothendieckTopology.{v, u} C
P : ObjectProperty (GrothendieckTopology.Point.{w, v, u} J)
inst_1 : LocallySmall.{w, v, u} C
hP :
  ∀ ⦃X : C⦄ (S : Sieve.{v, u} X),
    (∀ (Φ : P.FullSubcategory) (x : Φ.obj (P := P).fiber.obj X),
        ∃ (Y : C) (g : Y ⟶ X) (_ : S.arrows g) (y : Φ.obj (P := P).fiber.obj Y),
          (ConcreteCategory.hom (FC := TypeCat.Fun) (Φ.obj (P := P).fiber.map g) :
                (a : (fun (X : Type w) ↦ X) (Φ.obj.fiber.obj Y)) → (fun (X : Type w) ↦ X) (Φ.obj.fiber.obj X))
              y =
            x) →
      S ∈ (J : (a : C) → Set (Sieve a)) X
F₁ F₂ : Functor.{v, w, u, w + 1} Cᵒᵖ (Type w)
f : F₁ ⟶ F₂
inst_2 : Mono f
this :
  ∀ {F₁ F₂ : Functor.{v, w, u, w + 1} Cᵒᵖ (Type w)} (f : F₁ ⟶ F₂) [Mono f]
    (hf :
      ∀ (Φ : P.FullSubcategory),
        Function.Surjective (α := (fun (X : Type w) ↦ X) (Functor.obj Φ.obj (P := P).presheafFiber F₁)) (β :=
          (fun (X : Type w) ↦ X) (Functor.obj Φ.obj (P := P).presheafFiber F₂))
          (⇑(ConcreteCategory.hom (FC := TypeCat.Fun) (Functor.map Φ.obj (P := P).presheafFiber f)) :
            (a : (fun (X : Type w) ↦ X) (Φ.obj.presheafFiber.obj F₁)) →
              (fun (X : Type w) ↦ X) (Φ.obj.presheafFiber.obj F₂)))
    (hF₂ : ∃ (U : C), F₂ = Functor.obj shrinkYoneda.{w, v, u} U), Presheaf.IsLocallySurjective J (FA := TypeCat.Fun) f
hF₂ : ¬∃ (U : C), F₂ = Functor.obj shrinkYoneda.{w, v, u} U
U : C
s : ToType (FC := TypeCat.Fun) (F₂.obj (op U))
f' : pullback (Y := Functor.obj shrinkYoneda.{w, v, u} U) f
    ((Equiv.symm shrinkYonedaEquiv : (a : F₂.obj (op U)) → shrinkYoneda.{w, v, u}.obj U ⟶ F₂) s) ⟶
  Functor.obj shrinkYoneda.{w, v, u} U :=
  pullback.snd f ((Equiv.symm shrinkYonedaEquiv : (a : F₂.obj (op U)) → shrinkYoneda.{w, v, u}.obj U ⟶ F₂) s)
Φ : P.FullSubcategory
hf :
  Function.Surjective (α := (fun (X : Type w) ↦ X) (Functor.obj Φ.obj (P := P).presheafFiber F₁)) (β :=
    (fun (X : Type w) ↦ X) (Functor.obj Φ.obj (P := P).presheafFiber F₂))
    (⇑(ConcreteCategory.hom (FC := TypeCat.Fun) (Functor.map Φ.obj (P := P).presheafFiber f)) :
      (a : (fun (X : Type w) ↦ X) (Φ.obj.presheafFiber.obj F₁)) → (fun (X : Type w) ↦ X) (Φ.obj.presheafFiber.obj F₂))
⊢ Function.Surjective (α :=
    (fun (X : Type w) ↦ X)
      (Functor.obj Φ.obj (P := P).presheafFiber
        (pullback (Y := Functor.obj shrinkYoneda.{w, v, u} U) f
          ((Equiv.symm shrinkYonedaEquiv : (a : F₂.obj (op U)) → shrinkYoneda.{w, v, u}.obj U ⟶ F₂) s))))
    (β := (fun (X : Type w) ↦ X) (Functor.obj Φ.obj (P := P).presheafFiber (Functor.obj shrinkYoneda.{w, v, u} U)))
    (⇑(ConcreteCategory.hom (FC := TypeCat.Fun) (Functor.map Φ.obj (P := P).presheafFiber f')) :
      (a :
          (fun (X : Type w) ↦ X)
            (Φ.obj.presheafFiber.obj
              (pullback f ((shrinkYonedaEquiv.symm : (a : F₂.obj (op U)) → shrinkYoneda.{w, v, u}.obj U ⟶ F₂) s)))) →
        (fun (X : Type w) ↦ X) (Φ.obj.presheafFiber.obj (shrinkYoneda.{w, v, u}.obj U)))
have rw : ∀ (hf : Epi (X := Functor.obj Φ.obj (P := P).presheafFiber F₁) (Functor.map Φ.obj (P := P).presheafFiber f)),
  Epi (X :=
    Functor.obj Φ.obj (P := P).presheafFiber
      (pullback (Y := Functor.obj shrinkYoneda.{w, v, u} U) f
        ((Equiv.symm shrinkYonedaEquiv : (a : F₂.obj (op U)) → shrinkYoneda.{w, v, u}.obj U ⟶ F₂) s)))
    (Functor.map Φ.obj (P := P).presheafFiber f')
Tactic: rw [← CategoryTheory.epi_iff_surjective] at hf ⊢
Cleared hypotheses: hf