-- 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
V₁ : C
iVW : V₁ ⟶ W
iV₁Y₁ : V₁ ⟶ Y₁
e₁ : G.map iV₁Y₁ = G.map iVW ≫ i ≫ f₁
V₂ : C
iV₂V₁ : V₂ ⟶ V₁
iV₂Y₂ : V₂ ⟶ Y₂
e₂ : G.map iV₂Y₂ = G.map iV₂V₁ ≫ G.map iVW ≫ i ≫ f₂
V₃ : C
iV₃ : V₃ ⟶ V₂
e₄ : iV₃ ≫ iV₂V₁ ≫ iV₁Y₁ ≫ g₁ = iV₃ ≫ iV₂Y₂ ≫ g₂
⊢ Eq (α := (fun (X : Type u_5) ↦ X) (ℱ.obj (P := Presheaf.IsSheaf K).obj (op (G.obj V₃))))
    ((ConcreteCategory.hom (FC := TypeCat.Fun) (ℱ.obj (P := Presheaf.IsSheaf K).map (G.map iV₃).op) :
          (a : (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op (G.obj V₂)))) →
            (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op (G.obj V₃))))
        ((ConcreteCategory.hom (FC := TypeCat.Fun) (ℱ.obj (P := Presheaf.IsSheaf K).map (G.map iV₂V₁).op) :
              (a : (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op (G.obj V₁)))) →
                (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op (G.obj V₂))))
            ((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)))) :
          (fun (X : Type u_5) ↦ X) (ℱ.obj (P := Presheaf.IsSheaf K).obj (op (G.obj V₂)))) :
      (fun (X : Type u_5) ↦ X) (ℱ.obj (P := Presheaf.IsSheaf K).obj (op (G.obj V₃))))
    ((ConcreteCategory.hom (FC := TypeCat.Fun) (ℱ.obj (P := Presheaf.IsSheaf K).map (G.map iV₃).op) :
          (a : (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op (G.obj V₂)))) →
            (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op (G.obj V₃))))
        ((ConcreteCategory.hom (FC := TypeCat.Fun) (ℱ.obj (P := Presheaf.IsSheaf K).map (G.map iV₂V₁).op) :
              (a : (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op (G.obj V₁)))) →
                (fun (X : Type u_5) ↦ X) (ℱ.obj.obj (op (G.obj V₂))))
            ((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)))) :
          (fun (X : Type u_5) ↦ X) (ℱ.obj (P := Presheaf.IsSheaf K).obj (op (G.obj V₂)))) :
      (fun (X : Type u_5) ↦ X) (ℱ.obj (P := Presheaf.IsSheaf K).obj (op (G.obj V₃))))
have compatible : Eq (α := ℱ.obj (P := Presheaf.IsSheaf K).obj (op (G.obj V₃)))
  ((ConcreteCategory.hom (FC := TypeCat.Fun) (ℱ.obj (P := Presheaf.IsSheaf K).map (G.map (iV₃ ≫ iV₂V₁ ≫ iV₁Y₁)).op) :
        (a : ℱ.obj.obj (op (G.obj Y₁))) → ℱ.obj.obj (op (G.obj V₃)))
      (x g₁ hg₁) :
    ℱ.obj (P := Presheaf.IsSheaf K).obj (op (G.obj V₃)))
  ((ConcreteCategory.hom (FC := TypeCat.Fun) (ℱ.obj (P := Presheaf.IsSheaf K).map (G.map (iV₃ ≫ iV₂Y₂)).op) :
        (a : ℱ.obj.obj (op (G.obj Y₂))) → ℱ.obj.obj (op (G.obj V₃)))
      (x g₂ hg₂) :
    ℱ.obj (P := Presheaf.IsSheaf K).obj (op (G.obj V₃)))
Tactic: simp only [← op_comp, ← Functor.map_comp_apply, ← e₁, ← e₂, ← Functor.map_comp]