-- 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 : Functor.{v, w, u, w + 1} Cᵒᵖ (Type w)
U : C
f : F₁ ⟶ Functor.obj shrinkYoneda.{w, v, u} U
inst_2 : 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 (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 F₁)) →
          (fun (X : Type w) ↦ X) (Φ.obj.presheafFiber.obj (shrinkYoneda.{w, v, u}.obj U)))
Φ : P.FullSubcategory
u : Φ.obj (P := P).fiber.obj U
⊢ ∃ (Y : C) (g : Y ⟶ U) (_ :
    Sieve.arrows
      (Presheaf.imageSieve (FA := TypeCat.Fun) f
        ((Equiv.symm shrinkYonedaObjObjEquiv : (a : unop (op U) ⟶ U) → (shrinkYoneda.{w, v, u}.obj U).obj (op U))
          (𝟙 U)))
      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 U))
        y =
      u
have obtain : ∀ (x₁ : Functor.obj Φ.obj (P := P).presheafFiber F₁)
  (hx₁ :
    Eq (α := (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 F₁)) →
              (fun (X : Type w) ↦ X) (Φ.obj.presheafFiber.obj (shrinkYoneda.{w, v, u}.obj U)))
          x₁ :
        (fun (X : Type w) ↦ X) (Functor.obj Φ.obj (P := P).presheafFiber (Functor.obj shrinkYoneda.{w, v, u} U)))
      ((ConcreteCategory.hom (FC := TypeCat.Fun)
              (NatTrans.app (Iso.inv Φ.obj (P := P).shrinkYonedaCompPresheafFiberIso) U) :
            (a : (fun (X : Type w) ↦ X) (Φ.obj.fiber.obj U)) →
              (fun (X : Type w) ↦ X) ((shrinkYoneda.{w, v, u} ⋙ Φ.obj.presheafFiber).obj U))
          u :
        (fun (X : Type w) ↦ X) ((shrinkYoneda.{w, v, u} ⋙ Φ.obj (P := P).presheafFiber).obj U))),
  ∃ (Y : C) (g : Y ⟶ U) (_ :
    Sieve.arrows
      (Presheaf.imageSieve (FA := TypeCat.Fun) f
        ((Equiv.symm shrinkYonedaObjObjEquiv : (a : unop (op U) ⟶ U) → (shrinkYoneda.{w, v, u}.obj U).obj (op U))
          (𝟙 U)))
      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 U))
        y =
      u
Tactic: obtain ⟨x₁, hx₁⟩ := hf Φ (Φ.obj.shrinkYonedaCompPresheafFiberIso.inv.app _ u)