-- Lean 4 proof step from Mathlib.CategoryTheory.Yoneda (goal)
Goal:
C_1 : Type u₁
inst : Category.{v₁, u₁} C_1
C : Type u₁
inst_1 : Category.{v₁, u₁} C
X : Cᵒᵖ
Y Z : Cᵒᵖ ⥤ Type (max w v₁)
f : Y ⟶ Z
⊢ Eq (α :=
    obj (obj (Functor.op uliftYoneda.{w, v₁, u₁} ⋙ coyoneda) X) Y ⟶
      obj
        (obj
          (evaluation Cᵒᵖ (Type (max v₁ w)) ⋙
            obj (whiskeringRight (Cᵒᵖ ⥤ Type (max v₁ w)) (Type (max v₁ w)) (Type (max (max w u₁) v₁)))
              uliftFunctor.{u₁, max v₁ w})
          X)
        Z)
    (map (obj (Functor.op uliftYoneda.{w, v₁, u₁} ⋙ coyoneda) X) f ≫
      Iso.hom
        ((fun (x : Cᵒᵖ ⥤ Type (max w v₁)) ↦
            (Equiv.toIso (Equiv.trans uliftYonedaEquiv (Equiv.symm Equiv.ulift)) :
              obj (obj (Functor.op uliftYoneda.{w, v₁, u₁} ⋙ coyoneda) X) x ≅
                obj
                  (obj
                    (evaluation Cᵒᵖ (Type (max v₁ w)) ⋙
                      obj (whiskeringRight (Cᵒᵖ ⥤ Type (max v₁ w)) (Type (max v₁ w)) (Type (max (max w u₁) v₁)))
                        uliftFunctor.{u₁, max v₁ w})
                    X)
                  x))
          Z))
    (Iso.hom
        ((fun (x : Cᵒᵖ ⥤ Type (max w v₁)) ↦
            (Equiv.toIso (Equiv.trans uliftYonedaEquiv (Equiv.symm Equiv.ulift)) :
              obj (obj (Functor.op uliftYoneda.{w, v₁, u₁} ⋙ coyoneda) X) x ≅
                obj
                  (obj
                    (evaluation Cᵒᵖ (Type (max v₁ w)) ⋙
                      obj (whiskeringRight (Cᵒᵖ ⥤ Type (max v₁ w)) (Type (max v₁ w)) (Type (max (max w u₁) v₁)))
                        uliftFunctor.{u₁, max v₁ w})
                    X)
                  x))
          Y) ≫
      map
        (obj
          (evaluation Cᵒᵖ (Type (max v₁ w)) ⋙
            obj (whiskeringRight (Cᵒᵖ ⥤ Type (max v₁ w)) (Type (max v₁ w)) (Type (max (max w u₁) v₁)))
              uliftFunctor.{u₁, max v₁ w})
          X)
        f)
have h : ∀ (g : obj (obj (Functor.op uliftYoneda.{w, v₁, u₁} ⋙ coyoneda) X) Y),
  Eq (α :=
    obj
      (obj
        (evaluation Cᵒᵖ (Type (max v₁ w)) ⋙
          obj (whiskeringRight (Cᵒᵖ ⥤ Type (max v₁ w)) (Type (max v₁ w)) (Type (max (max w u₁) v₁)))
            uliftFunctor.{u₁, max v₁ w})
        X)
      Z)
    (TypeCat.Fun.toFun
      (ConcreteCategory.hom (FC := TypeCat.Fun)
        (map (obj (Functor.op uliftYoneda.{w, v₁, u₁} ⋙ coyoneda) X) f ≫
          Iso.hom
            ((fun (x : Cᵒᵖ ⥤ Type (max w v₁)) ↦
                (Equiv.toIso (Equiv.trans uliftYonedaEquiv (Equiv.symm Equiv.ulift)) :
                  obj (obj (Functor.op uliftYoneda.{w, v₁, u₁} ⋙ coyoneda) X) x ≅
                    obj
                      (obj
                        (evaluation Cᵒᵖ (Type (max v₁ w)) ⋙
                          obj (whiskeringRight (Cᵒᵖ ⥤ Type (max v₁ w)) (Type (max v₁ w)) (Type (max (max w u₁) v₁)))
                            uliftFunctor.{u₁, max v₁ w})
                        X)
                      x))
              Z)))
      g)
    (TypeCat.Fun.toFun
      (ConcreteCategory.hom (FC := TypeCat.Fun)
        (Iso.hom
            ((fun (x : Cᵒᵖ ⥤ Type (max w v₁)) ↦
                (Equiv.toIso (Equiv.trans uliftYonedaEquiv (Equiv.symm Equiv.ulift)) :
                  obj (obj (Functor.op uliftYoneda.{w, v₁, u₁} ⋙ coyoneda) X) x ≅
                    obj
                      (obj
                        (evaluation Cᵒᵖ (Type (max v₁ w)) ⋙
                          obj (whiskeringRight (Cᵒᵖ ⥤ Type (max v₁ w)) (Type (max v₁ w)) (Type (max (max w u₁) v₁)))
                            uliftFunctor.{u₁, max v₁ w})
                        X)
                      x))
              Y) ≫
          map
            (obj
              (evaluation Cᵒᵖ (Type (max v₁ w)) ⋙
                obj (whiskeringRight (Cᵒᵖ ⥤ Type (max v₁ w)) (Type (max v₁ w)) (Type (max (max w u₁) v₁)))
                  uliftFunctor.{u₁, max v₁ w})
              X)
            f))
      g)
Tactic: ext g