-- Lean 4 proof step from Mathlib.CategoryTheory.Topos.Sheaf (goal)
Goal:
C : Type u
inst : Category.{v, u} C
J : GrothendieckTopology.{v, u} C
F G : Cᵒᵖ ⥤ Type (max u v)
m : F ⟶ G
inst_1 : Mono m
hF : Presieve.IsSheaf J F
hG : Presieve.IsSeparated J G
X : Cᵒᵖ
x : G.obj X
Y : C
f : Y ⟶ Opposite.unop X
hf :
  J.Covers
    ((ConcreteCategory.hom (FC := TypeCat.Fun)
          (↾fun (x : G.obj X) ↦
            {
              arrows := fun (Y : C) (f : Y ⟶ Opposite.unop X) ↦
                ∃ (a : F.obj (Opposite.op Y)),
                  Eq (α := G.obj (Opposite.op Y))
                    ((ConcreteCategory.hom (FC := TypeCat.Fun) (G.map f.op) : (a : G.obj X) → G.obj (Opposite.op Y)) x :
                      G.obj (Opposite.op Y))
                    ((ConcreteCategory.hom (FC := TypeCat.Fun) (m.app (Opposite.op Y)) :
                          (a : F.obj (Opposite.op Y)) → G.obj (Opposite.op Y))
                        a :
                      G.obj (Opposite.op Y)),
              downward_closed := ⋯ : Sieve.{v, u} (Opposite.unop X) }) :
        (a : G.obj X) → (Functor.sieves C).obj X)
      x)
    f
a :
  (Z : C) →
    (g : Z ⟶ Y) →
      (hg :
          (Sieve.pullback f
                ((ConcreteCategory.hom (FC := TypeCat.Fun) ((χ m).app X) :
                    (a : (fun (X : Type (max u v)) ↦ X) (G.obj X)) →
                      (fun (X : Type (max u v)) ↦ X) ((Functor.sieves C).obj X))
                  x)).arrows
            g) →
        (fun (X : Type (max u v)) ↦ X) (F.obj (Opposite.op Z))
ha :
  ∀ (Z : C) (g : Z ⟶ Y)
    (hg :
      (Sieve.pullback f
            ((ConcreteCategory.hom (FC := TypeCat.Fun) ((χ m).app X) :
                (a : (fun (X : Type (max u v)) ↦ X) (G.obj X)) →
                  (fun (X : Type (max u v)) ↦ X) ((Functor.sieves C).obj X))
              x)).arrows
        g),
    Eq (α := (fun (X : Type (max u v)) ↦ X) (G.obj (Opposite.op Z)))
      ((ConcreteCategory.hom (FC := TypeCat.Fun) (G.map (g ≫ f).op) :
            (a : (fun (X : Type (max u v)) ↦ X) (G.obj (Opposite.op (Opposite.unop X)))) →
              (fun (X : Type (max u v)) ↦ X) (G.obj (Opposite.op Z)))
          x :
        (fun (X : Type (max u v)) ↦ X) (G.obj (Opposite.op Z)))
      ((ConcreteCategory.hom (FC := TypeCat.Fun) (m.app (Opposite.op Z)) :
            (a : (fun (X : Type (max u v)) ↦ X) (F.obj (Opposite.op Z))) →
              (fun (X : Type (max u v)) ↦ X) (G.obj (Opposite.op Z)))
          (a Z g hg) :
        (fun (X : Type (max u v)) ↦ X) (G.obj (Opposite.op Z)))
refine_1 :
  Presieve.FamilyOfElements.Compatible (R :=
    (Sieve.pullback f
        ((ConcreteCategory.hom (FC := TypeCat.Fun)
              (↾fun (x : G.obj X) ↦
                {
                  arrows := fun (Y : C) (f : Y ⟶ Opposite.unop X) ↦
                    ∃ (a : F.obj (Opposite.op Y)),
                      Eq (α := G.obj (Opposite.op Y))
                        ((ConcreteCategory.hom (FC := TypeCat.Fun) (G.map f.op) : (a : G.obj X) → G.obj (Opposite.op Y))
                            x :
                          G.obj (Opposite.op Y))
                        ((ConcreteCategory.hom (FC := TypeCat.Fun) (m.app (Opposite.op Y)) :
                              (a : F.obj (Opposite.op Y)) → G.obj (Opposite.op Y))
                            a :
                          G.obj (Opposite.op Y)),
                  downward_closed := ⋯ : Sieve.{v, u} (Opposite.unop X) }) :
            (a : G.obj X) → (Functor.sieves C).obj X)
          x)).arrows)
    a
⊢ Sieve.arrows
    ((ConcreteCategory.hom (FC := TypeCat.Fun)
          (↾fun (x : G.obj X) ↦
            {
              arrows := fun (Y : C) (f : Y ⟶ Opposite.unop X) ↦
                ∃ (a : F.obj (Opposite.op Y)),
                  Eq (α := G.obj (Opposite.op Y))
                    ((ConcreteCategory.hom (FC := TypeCat.Fun) (G.map f.op) : (a : G.obj X) → G.obj (Opposite.op Y)) x :
                      G.obj (Opposite.op Y))
                    ((ConcreteCategory.hom (FC := TypeCat.Fun) (m.app (Opposite.op Y)) :
                          (a : F.obj (Opposite.op Y)) → G.obj (Opposite.op Y))
                        a :
                      G.obj (Opposite.op Y)),
              downward_closed := ⋯ : Sieve.{v, u} (Opposite.unop X) }) :
        (a : G.obj X) → (Functor.sieves C).obj X)
      x)
    f
have refine_2 : Eq (α := G.obj (Opposite.op Y))
  ((ConcreteCategory.hom (FC := TypeCat.Fun) (G.map f.op) : (a : G.obj X) → G.obj (Opposite.op Y)) x :
    G.obj (Opposite.op Y))
  ((ConcreteCategory.hom (FC := TypeCat.Fun) (m.app (Opposite.op Y)) :
        (a : F.obj (Opposite.op Y)) → G.obj (Opposite.op Y))
      (sorry.amalgamate (R :=
        (Sieve.pullback f
            ((ConcreteCategory.hom (FC := TypeCat.Fun)
                  (↾fun (x : G.obj X) ↦
                    {
                      arrows := fun (Y : C) (f : Y ⟶ Opposite.unop X) ↦
                        ∃ (a : F.obj (Opposite.op Y)),
                          Eq (α := G.obj (Opposite.op Y))
                            ((ConcreteCategory.hom (FC := TypeCat.Fun) (G.map f.op) :
                                  (a : G.obj X) → G.obj (Opposite.op Y))
                                x :
                              G.obj (Opposite.op Y))
                            ((ConcreteCategory.hom (FC := TypeCat.Fun) (m.app (Opposite.op Y)) :
                                  (a : F.obj (Opposite.op Y)) → G.obj (Opposite.op Y))
                                a :
                              G.obj (Opposite.op Y)),
                      downward_closed := sorry : Sieve.{v, u} (Opposite.unop X) }) :
                (a : G.obj X) → (Functor.sieves C).obj X)
              x)).arrows)
        a sorry) :
    G.obj (Opposite.op Y))
Tactic: refine ⟨(hF _ hf).amalgamate a ?_, ?_⟩