-- Lean 4 proof step from Mathlib.CategoryTheory.Sites.DenseSubsite.SheafEquiv (goal)
Goal:
C : Type u_1
D : Type u_2
inst : Category.{v_1, u_1} C
inst_1 : Category.{v_2, u_2} D
G : Functor.{v_1, v_2, u_1, u_2} C D
J : GrothendieckTopology.{v_1, u_1} C
K : GrothendieckTopology.{v_2, u_2} D
A : Type w
inst_2 : Category.{w', w} A
inst_3 : ∀ (X : Dᵒᵖ), Limits.HasLimitsOfShape (StructuredArrow X G.op) A
inst_4 : IsDenseSubsite J K G
Y : Sheaf.{v_1, w', u_1, w} J A
U : C
X : A
f₁ f₂ :
  obj (obj yoneda ((obj (G.op.ran ⋙ obj (whiskeringLeft Cᵒᵖ Dᵒᵖ A) G.op) (Y.obj (P := Presheaf.IsSheaf J))).obj (op U)))
    (op X)
e :
  Eq (α :=
    (fun (X : Type w') ↦ X)
      (obj (obj yoneda ((obj (𝟭 (Functor.{v_1, w', u_1, w} Cᵒᵖ A)) (Y.obj (P := Presheaf.IsSheaf J))).obj (op U)))
        (op X)))
    ((ConcreteCategory.hom (FC := TypeCat.Fun)
            (NatTrans.app
              (map yoneda (NatTrans.app (NatTrans.app G.op.ranCounit (Y.obj (P := Presheaf.IsSheaf J))) (op U)))
              (op X)) :
          (a :
              (fun (X : Type w') ↦ X)
                ((yoneda.obj (((G.op.ran ⋙ (whiskeringLeft Cᵒᵖ Dᵒᵖ A).obj G.op).obj Y.obj).obj (op U))).obj (op X))) →
            (fun (X : Type w') ↦ X) ((yoneda.obj (((𝟭 (Cᵒᵖ ⥤ A)).obj Y.obj).obj (op U))).obj (op X)))
        f₁ :
      (fun (X : Type w') ↦ X)
        (obj (obj yoneda ((obj (𝟭 (Functor.{v_1, w', u_1, w} Cᵒᵖ A)) (Y.obj (P := Presheaf.IsSheaf J))).obj (op U)))
          (op X)))
    ((ConcreteCategory.hom (FC := TypeCat.Fun)
            (NatTrans.app
              (map yoneda (NatTrans.app (NatTrans.app G.op.ranCounit (Y.obj (P := Presheaf.IsSheaf J))) (op U)))
              (op X)) :
          (a :
              (fun (X : Type w') ↦ X)
                ((yoneda.obj (((G.op.ran ⋙ (whiskeringLeft Cᵒᵖ Dᵒᵖ A).obj G.op).obj Y.obj).obj (op U))).obj (op X))) →
            (fun (X : Type w') ↦ X) ((yoneda.obj (((𝟭 (Cᵒᵖ ⥤ A)).obj Y.obj).obj (op U))).obj (op X)))
        f₂ :
      (fun (X : Type w') ↦ X)
        (obj (obj yoneda ((obj (𝟭 (Functor.{v_1, w', u_1, w} Cᵒᵖ A)) (Y.obj (P := Presheaf.IsSheaf J))).obj (op U)))
          (op X)))
W : C
g : G.obj W ⟶ G.obj U
V : C
iVW : V ⟶ W
h_1 : (G.imageSieve g).arrows iVW
iVU : V ⟶ U
h : G.map iVU = G.map iVW ≫ g
this :
  (f₁ ≫ NatTrans.app (NatTrans.app G.op.ranCounit (Y.obj (P := Presheaf.IsSheaf J))) (op U)) ≫
      Y.obj (P := Presheaf.IsSheaf J).map iVU.op =
    (f₂ ≫ NatTrans.app (NatTrans.app G.op.ranCounit (Y.obj (P := Presheaf.IsSheaf J))) (op U)) ≫
      Y.obj (P := Presheaf.IsSheaf J).map iVU.op
⊢ (f₁ ≫
        (obj G.op.ran (Y.obj (P := Presheaf.IsSheaf J))).map (op g) ≫
          NatTrans.app (NatTrans.app G.op.ranCounit (Y.obj (P := Presheaf.IsSheaf J))) (op W)) ≫
      Y.obj (P := Presheaf.IsSheaf J).map iVW.op =
    (f₂ ≫
        (obj G.op.ran (Y.obj (P := Presheaf.IsSheaf J))).map (op g) ≫
          NatTrans.app (NatTrans.app G.op.ranCounit (Y.obj (P := Presheaf.IsSheaf J))) (op W)) ≫
      Y.obj (P := Presheaf.IsSheaf J).map iVW.op
have left : ∀
  (this :
    f₁ ≫
        (G.op ⋙ obj G.op.ran (Y.obj (P := Presheaf.IsSheaf J))).map iVU.op ≫
          NatTrans.app (NatTrans.app G.op.ranCounit (Y.obj (P := Presheaf.IsSheaf J))) (op V) =
      f₂ ≫
        (G.op ⋙ obj G.op.ran (Y.obj (P := Presheaf.IsSheaf J))).map iVU.op ≫
          NatTrans.app (NatTrans.app G.op.ranCounit (Y.obj (P := Presheaf.IsSheaf J))) (op V)),
  f₁ ≫
      (obj G.op.ran (Y.obj (P := Presheaf.IsSheaf J))).map (op g) ≫
        (G.op ⋙ obj G.op.ran (Y.obj (P := Presheaf.IsSheaf J))).map iVW.op ≫
          NatTrans.app (NatTrans.app G.op.ranCounit (Y.obj (P := Presheaf.IsSheaf J))) (op V) =
    f₂ ≫
      (obj G.op.ran (Y.obj (P := Presheaf.IsSheaf J))).map (op g) ≫
        (G.op ⋙ obj G.op.ran (Y.obj (P := Presheaf.IsSheaf J))).map iVW.op ≫
          NatTrans.app (NatTrans.app G.op.ranCounit (Y.obj (P := Presheaf.IsSheaf J))) (op V)
Tactic: simp only [Category.assoc, ← NatTrans.naturality] at this ⊢
Cleared hypotheses: this