-- Lean 4 proof step from Mathlib.CategoryTheory.Limits.Presheaf (hypothesis)
Goal:
C : Type u₁
inst : Category.{v₁, u₁} C
ℰ : Type u₂
inst_1 : Category.{v₂, u₂} ℰ
A_1 A : Functor.{v₁, v₂, u₁, u₂} C ℰ
inst_2 : Functor.HasPointwiseLeftKanExtension uliftYoneda.{max w v₂, v₁, u₁} A
D : Type u₂
inst_3 : Category.{v₂, u₂} D
F : Functor.{v₁, v₂, u₁, u₂} C D
inst_4 : ∀ (P : Cᵒᵖ ⥤ Type (max w v₁ v₂)), F.op.HasLeftKanExtension P
X Y : C
f : X ⟶ Y
⊢ Eq (α := ((uliftYoneda.{max w v₂, v₁, u₁} ⋙ F.op.lan).obj Y).obj (op (F.obj X)))
    ((uliftYonedaEquiv :
          (a : uliftYoneda.{max v₁ w, v₂, u₂}.obj (F.obj X) ⟶ (uliftYoneda.{max w v₂, v₁, u₁} ⋙ F.op.lan).obj Y) →
            ((uliftYoneda.{max w v₂, v₁, u₁} ⋙ F.op.lan).obj Y).obj (op (F.obj X)))
        ((F ⋙ uliftYoneda.{max w v₁, v₂, u₂}).map f ≫
          Iso.hom
            ((fun (X : C) ↦
                (Functor.leftKanExtensionUnique (Functor.obj uliftYoneda.{max w v₁, v₂, u₂} (F.obj X))
                    (uliftYonedaMap.{w, v₁, v₂, u₁, u₂} F X)
                    (Functor.obj F.op.lan (Functor.obj uliftYoneda.{max w v₂, v₁, u₁} X))
                    (NatTrans.app F.op.lanUnit (Functor.obj uliftYoneda.{max v₂ w, v₁, u₁} X)) :
                  Functor.obj uliftYoneda.{max w v₁, v₂, u₂} (F.obj X) ≅
                    Functor.obj F.op.lan (Functor.obj uliftYoneda.{max w v₂, v₁, u₁} X)))
              Y)) :
      ((uliftYoneda.{max w v₂, v₁, u₁} ⋙ F.op.lan).obj Y).obj (op (F.obj X)))
    ((uliftYonedaEquiv :
          (a : uliftYoneda.{max v₁ w, v₂, u₂}.obj (F.obj X) ⟶ (uliftYoneda.{max w v₂, v₁, u₁} ⋙ F.op.lan).obj Y) →
            ((uliftYoneda.{max w v₂, v₁, u₁} ⋙ F.op.lan).obj Y).obj (op (F.obj X)))
        (Iso.hom
            ((fun (X : C) ↦
                (Functor.leftKanExtensionUnique (Functor.obj uliftYoneda.{max w v₁, v₂, u₂} (F.obj X))
                    (uliftYonedaMap.{w, v₁, v₂, u₁, u₂} F X)
                    (Functor.obj F.op.lan (Functor.obj uliftYoneda.{max w v₂, v₁, u₁} X))
                    (NatTrans.app F.op.lanUnit (Functor.obj uliftYoneda.{max v₂ w, v₁, u₁} X)) :
                  Functor.obj uliftYoneda.{max w v₁, v₂, u₂} (F.obj X) ≅
                    Functor.obj F.op.lan (Functor.obj uliftYoneda.{max w v₂, v₁, u₁} X)))
              X) ≫
          (uliftYoneda.{max w v₂, v₁, u₁} ⋙ F.op.lan).map f) :
      ((uliftYoneda.{max w v₂, v₁, u₁} ⋙ F.op.lan).obj Y).obj (op (F.obj X)))
have eq₁ : Eq (α := (Functor.obj F.op.lan (Functor.obj uliftYoneda.{max v₂ w, v₁, u₁} Y)).obj (F.op.obj (op X)))
  ((hom (FC := TypeCat.Fun)
          ((uliftYonedaMap.{w, v₁, v₂, u₁, u₂} F Y).app (op X) ≫
            NatTrans.app
              (Functor.descOfIsLeftKanExtension (Functor.obj uliftYoneda.{max w v₁, v₂, u₂} (F.obj Y))
                (uliftYonedaMap.{w, v₁, v₂, u₁, u₂} F Y)
                (Functor.obj F.op.lan (Functor.obj uliftYoneda.{max v₂ w, v₁, u₁} Y))
                (NatTrans.app F.op.lanUnit (Functor.obj uliftYoneda.{max v₂ w, v₁, u₁} Y)))
              (F.op.obj (op X))) :
        (a : (uliftYoneda.{max w v₂, v₁, u₁}.obj Y).obj (op X)) →
          (F.op.lan.obj (uliftYoneda.{max v₂ w, v₁, u₁}.obj Y)).obj (F.op.obj (op X)))
      { down := f } :
    (Functor.obj F.op.lan (Functor.obj uliftYoneda.{max v₂ w, v₁, u₁} Y)).obj (F.op.obj (op X)))
  ((hom (FC := TypeCat.Fun)
          (NatTrans.app (NatTrans.app F.op.lanUnit (Functor.obj uliftYoneda.{max v₂ w, v₁, u₁} Y)) (op X)) :
        (a : (uliftYoneda.{max w v₂, v₁, u₁}.obj Y).obj (op X)) →
          (F.op.lan.obj (uliftYoneda.{max v₂ w, v₁, u₁}.obj Y)).obj (F.op.obj (op X)))
      { down := f } :
    (Functor.obj F.op.lan (Functor.obj uliftYoneda.{max v₂ w, v₁, u₁} Y)).obj (F.op.obj (op X)))
Tactic: have eq₁ := ConcreteCategory.congr_hom
        ((uliftYoneda.{max w v₁}.obj (F.obj Y)).descOfIsLeftKanExtension_fac_app
        (uliftYonedaMap F Y) (F.op.lan.obj (uliftYoneda.obj Y))
          (F.op.lanUnit.app (uliftYoneda.obj Y)) _) ⟨f⟩