-- Lean 4 proof step from Mathlib.CategoryTheory.Sites.Subcanonical (goal)
Goal:
C : Type u
inst : Category.{v, u} C
J : GrothendieckTopology.{v, u} C
inst_1 : J.Subcanonical
ι : Type u_1
X : ι → C
c : Cofan.{u_1, v, u} X
H : Membership.mem (γ := Set (Sieve.{v, u} c.pt)) ((J : (a : C) → Set (Sieve a)) c.pt) (Sieve.ofArrows X c.inj)
inst_2 : ∀ (i : ι), Mono (c.inj i)
hempty : ∀ (Y : C) (a : IsInitial.{v, u} Y), ⊥ ∈ (J : (a : C) → Set (Sieve a)) Y
hdisj :
  ∀ {i j : ι}, i ≠ j → ∀ {Y : C} (a : Y ⟶ X i) (b : Y ⟶ X j), a ≫ c.inj i = b ≫ c.inj j → Nonempty (IsInitial.{v, u} Y)
heq :
  ∀ (s : Cofan (C := Sheaf J (Type v)) fun (i : ι) ↦ J.yoneda.obj (X i)) {Y : C} {i j : ι} (a : Y ⟶ X i) (b : Y ⟶ X j)
    (hab : a ≫ c.inj i = b ≫ c.inj j),
    Eq (α := (fun (X : Type v) ↦ X) (s.pt.obj (P := Presheaf.IsSheaf J).obj (op Y)))
      ((ConcreteCategory.hom (FC := TypeCat.Fun) ((s.inj i).hom.app (op Y)) :
            (a : (fun (X : Type v) ↦ X) ((J.yoneda.obj (X i)).obj.obj (op Y))) →
              (fun (X : Type v) ↦ X) (s.pt.obj.obj (op Y)))
          a :
        (fun (X : Type v) ↦ X) (s.pt.obj (P := Presheaf.IsSheaf J).obj (op Y)))
      ((ConcreteCategory.hom (FC := TypeCat.Fun) ((s.inj j).hom.app (op Y)) :
            (a : (fun (X : Type v) ↦ X) ((J.yoneda.obj (X j)).obj.obj (op Y))) →
              (fun (X : Type v) ↦ X) (s.pt.obj.obj (op Y)))
          b :
        (fun (X : Type v) ↦ X) (s.pt.obj (P := Presheaf.IsSheaf J).obj (op Y)))
s : Cofan (C := Sheaf J (Type v)) fun (i : ι) ↦ J.yoneda.obj (X i)
j : ι
u : (j : ι) → obj CategoryTheory.yoneda (X j) ⟶ (Sieve.ofArrows X c.inj).functor :=
  fun (j : ι) ↦ (Sieve.ofArrows X c.inj).toFunctor (c.inj j) ⋯
this : ∀ (j : ι), u j ≫ (Sieve.ofArrows X c.inj).functorInclusion = map CategoryTheory.yoneda (c.inj j)
⊢ u j ≫
      {
        app := fun (Y : Cᵒᵖ) ↦
          ↾fun
              (g :
                { g : unop Y ⟶ c.pt //
                  ∃ (Y_1 : C) (h : unop Y ⟶ Y_1) (g_1 : Y_1 ⟶ c.pt), Presieve.ofArrows X c.inj g_1 ∧ h ≫ g_1 = g }) ↦
            (ConcreteCategory.hom (FC := TypeCat.Fun)
                  (NatTrans.app (InducedCategory.Hom.hom (s.inj (Sieve.ofArrows.i ⋯))) Y) :
                (a : unop Y ⟶ X (Sieve.ofArrows.i ⋯)) → s.pt.obj.obj Y)
              (Sieve.ofArrows.h ⋯),
        naturality := ⋯ } =
    (s.inj j).hom
have refine_2 : ∀ (Z : Cᵒᵖ) (g : unop Z ⟶ X j),
  TypeCat.Fun.toFun
      (ConcreteCategory.hom (FC := TypeCat.Fun)
        (NatTrans.app
          (u j ≫
            {
              app := fun (Y : Cᵒᵖ) ↦
                ↾fun
                    (g :
                      { g : unop Y ⟶ c.pt //
                        ∃ (Y_1 : C) (h : unop Y ⟶ Y_1) (g_1 : Y_1 ⟶ c.pt),
                          Presieve.ofArrows X c.inj g_1 ∧ h ≫ g_1 = g }) ↦
                  (ConcreteCategory.hom (FC := TypeCat.Fun)
                        (NatTrans.app (InducedCategory.Hom.hom (s.inj (Sieve.ofArrows.i sorry))) Y) :
                      (a : unop Y ⟶ X (Sieve.ofArrows.i sorry)) → s.pt.obj.obj Y)
                    (Sieve.ofArrows.h sorry),
              naturality := sorry : NatTrans (Sieve.ofArrows X c.inj).functor (s.pt.obj (P := Presheaf.IsSheaf J)) })
          Z))
      g =
    (ConcreteCategory.hom (FC := TypeCat.Fun) ((s.inj j).hom.app Z)).toFun g
Tactic: ext Z (g : Z.unop ⟶ X j)