-- Lean 4 proof step from Mathlib.CategoryTheory.Sites.DenseSubsite.Basic (goal)
Goal:
C : Type u_1
inst : Category.{v_1, u_1} C
D : Type u_2
inst_1 : Category.{v_2, u_2} D
K : GrothendieckTopology.{v_2, u_2} D
G : Functor.{v_1, v_2, u_1, u_2} C D
inst_2 : G.IsCoverDense K
inst_3 : G.IsLocallyFull K
inst_4 : G.IsLocallyFaithful K
ℱ : Sheaf K (Type u_5)
Z : C
T : Presieve.{v_1, u_1} Z
x : FamilyOfElements (G.op ⋙ ℱ.obj (P := Presheaf.IsSheaf K)) T
hx : x.Compatible (R := T)
Y₁ Y₂ : C
X : D
f₁ : X ⟶ G.obj Y₁
f₂ : X ⟶ G.obj Y₂
g₁ : Y₁ ⟶ Z
g₂ : Y₂ ⟶ Z
hg₁ : T g₁
hg₂ : T g₂
eq : f₁ ≫ G.map g₁ = f₂ ≫ G.map g₂
W : C
i : G.obj W ⟶ X
⊢ Eq (α := (fun (X : Type u_5) ↦ X) (ℱ.obj (P := Presheaf.IsSheaf K).obj (op (G.obj W))))
    ((ConcreteCategory.hom (FC := TypeCat.Fun) (ℱ.obj (P := Presheaf.IsSheaf K).map i.op) :
          (a : (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op X))) → (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op (G.obj W))))
        ((ConcreteCategory.hom (FC := TypeCat.Fun) (ℱ.obj (P := Presheaf.IsSheaf K).map f₁.op) :
              (a : (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op (G.obj Y₁)))) → (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op X)))
            (x g₁ hg₁) :
          (fun (X : Type u_5) ↦ X) (ℱ.obj (P := Presheaf.IsSheaf K).obj (op X))) :
      (fun (X : Type u_5) ↦ X) (ℱ.obj (P := Presheaf.IsSheaf K).obj (op (G.obj W))))
    ((ConcreteCategory.hom (FC := TypeCat.Fun) (ℱ.obj (P := Presheaf.IsSheaf K).map i.op) :
          (a : (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op X))) → (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op (G.obj W))))
        ((ConcreteCategory.hom (FC := TypeCat.Fun) (ℱ.obj (P := Presheaf.IsSheaf K).map f₂.op) :
              (a : (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op (G.obj Y₂)))) → (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op X)))
            (x g₂ hg₂) :
          (fun (X : Type u_5) ↦ X) (ℱ.obj (P := Presheaf.IsSheaf K).obj (op X))) :
      (fun (X : Type u_5) ↦ X) (ℱ.obj (P := Presheaf.IsSheaf K).obj (op (G.obj W))))
have compatible : ∀ (V₁ : C) (iVW : V₁ ⟶ W) (iV₁Y₁ : V₁ ⟶ Y₁) (e₁ : G.map iV₁Y₁ = G.map iVW ≫ i ≫ f₁),
  Eq (α := (fun (X : Type u_5) ↦ X) (ℱ.obj (P := Presheaf.IsSheaf K).obj (op (G.obj V₁))))
    ((ConcreteCategory.hom (FC := TypeCat.Fun) (ℱ.obj (P := Presheaf.IsSheaf K).map (G.map iVW).op) :
          (a : (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op (G.obj W)))) →
            (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op (G.obj V₁))))
        ((ConcreteCategory.hom (FC := TypeCat.Fun) (ℱ.obj (P := Presheaf.IsSheaf K).map i.op) :
              (a : (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op X))) → (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op (G.obj W))))
            ((ConcreteCategory.hom (FC := TypeCat.Fun) (ℱ.obj (P := Presheaf.IsSheaf K).map f₁.op) :
                  (a : (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op (G.obj Y₁)))) →
                    (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op X)))
                (x g₁ hg₁) :
              (fun (X : Type u_5) ↦ X) (ℱ.obj (P := Presheaf.IsSheaf K).obj (op X))) :
          (fun (X : Type u_5) ↦ X) (ℱ.obj (P := Presheaf.IsSheaf K).obj (op (G.obj W)))) :
      (fun (X : Type u_5) ↦ X) (ℱ.obj (P := Presheaf.IsSheaf K).obj (op (G.obj V₁))))
    ((ConcreteCategory.hom (FC := TypeCat.Fun) (ℱ.obj (P := Presheaf.IsSheaf K).map (G.map iVW).op) :
          (a : (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op (G.obj W)))) →
            (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op (G.obj V₁))))
        ((ConcreteCategory.hom (FC := TypeCat.Fun) (ℱ.obj (P := Presheaf.IsSheaf K).map i.op) :
              (a : (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op X))) → (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op (G.obj W))))
            ((ConcreteCategory.hom (FC := TypeCat.Fun) (ℱ.obj (P := Presheaf.IsSheaf K).map f₂.op) :
                  (a : (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op (G.obj Y₂)))) →
                    (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op X)))
                (x g₂ hg₂) :
              (fun (X : Type u_5) ↦ X) (ℱ.obj (P := Presheaf.IsSheaf K).obj (op X))) :
          (fun (X : Type u_5) ↦ X) (ℱ.obj (P := Presheaf.IsSheaf K).obj (op (G.obj W)))) :
      (fun (X : Type u_5) ↦ X) (ℱ.obj (P := Presheaf.IsSheaf K).obj (op (G.obj V₁))))
Tactic: refine IsLocallyFull.ext G _ (i ≫ f₁) fun V₁ iVW iV₁Y₁ e₁ ↦ ?_