-- Lean 4 proof step from Mathlib.CategoryTheory.Galois.Prorepresentability (goal)
Goal:
C : Type u₁
inst : Category.{u₂, u₁} C
inst_1 : GaloisCategory.{u₁, u₂} C
F : C ⥤ FintypeCat
x x_1 : (PointedGaloisObject F)ᵒᵖ
A : C
a : (F.obj A).obj
isGalois : IsGalois.{u₁, u₂} A
B : C
b : (F.obj B).obj
isGalois_1 : IsGalois.{u₁, u₂} B
x_2 :
  Opposite.op { obj := A, pt := a, isGalois := isGalois : PointedGaloisObject F } ⟶
    Opposite.op { obj := B, pt := b, isGalois := isGalois_1 }
f :
  (Opposite.unop (Opposite.op { obj := B, pt := b, isGalois := isGalois_1 : PointedGaloisObject F })).obj ⟶
    (Opposite.unop (Opposite.op { obj := A, pt := a, isGalois := isGalois : PointedGaloisObject F })).obj
hf :
  (@ConcreteCategory.hom _ (ObjectProperty.FullSubcategory.category Finite)
          (fun (X Y : ObjectProperty.FullSubcategory Finite) ↦ TypeCat.Fun X.obj Y.obj)
          (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj)
          (fun (X Y : ObjectProperty.FullSubcategory Finite) ↦
            (TypeCat.instFunLikeFun : FunLike (TypeCat.Fun X.obj Y.obj) X.obj Y.obj))
          _ _ _ (F.map f) :
        (a_1 :
            (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj)
              (F.obj (Opposite.unop (Opposite.op { obj := B, pt := b, isGalois := isGalois_1 })).obj)) →
          (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj)
            (F.obj (Opposite.unop (Opposite.op { obj := A, pt := a, isGalois := isGalois })).obj))
      b =
    a
⊢ Eq (α :=
    Functor.obj (Functor.op (incl F) ⋙ coyoneda)
        (Opposite.op { obj := A, pt := a, isGalois := isGalois : PointedGaloisObject F }) ⟶
      Functor.obj (Functor.obj (const (PointedGaloisObject F)ᵒᵖ) (F ⋙ FintypeCat.incl))
        (Opposite.op { obj := B, pt := b, isGalois := isGalois_1 : PointedGaloisObject F }))
    (map (Functor.op (incl F) ⋙ coyoneda)
        (Opposite.op
          { val := f, comp := hf :
            (Opposite.unop (Opposite.op { obj := B, pt := b, isGalois := isGalois_1 : PointedGaloisObject F })).Hom
              (Opposite.unop (Opposite.op { obj := A, pt := a, isGalois := isGalois : PointedGaloisObject F })) }) ≫
      match (motive :=
        (x : (PointedGaloisObject F)ᵒᵖ) →
          Functor.obj (Functor.op (incl F) ⋙ coyoneda) x ⟶
            Functor.obj (Functor.obj (const (PointedGaloisObject F)ᵒᵖ) (F ⋙ FintypeCat.incl)) x)
        Opposite.op { obj := B, pt := b, isGalois := isGalois_1 : PointedGaloisObject F } with
      | Opposite.op { obj := A, pt := a, isGalois := isGalois } =>
        {
          app := fun (X : C) ↦
            ↾fun (f : A ⟶ X) ↦
              (@ConcreteCategory.hom _ (ObjectProperty.FullSubcategory.category Finite)
                    (fun (X Y : ObjectProperty.FullSubcategory Finite) ↦ TypeCat.Fun X.obj Y.obj)
                    (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj)
                    (fun (X Y : ObjectProperty.FullSubcategory Finite) ↦
                      (TypeCat.instFunLikeFun : FunLike (TypeCat.Fun X.obj Y.obj) X.obj Y.obj))
                    _ _ _ (F.map f) :
                  (a : (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A)) →
                    (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj X))
                a,
          naturality := ⋯ })
    (CategoryStruct.comp (Y :=
      Functor.obj (Functor.obj (const (PointedGaloisObject F)ᵒᵖ) (F ⋙ FintypeCat.incl))
        (Opposite.op { obj := A, pt := a, isGalois := isGalois : PointedGaloisObject F }))
      (match (motive :=
        (x : (PointedGaloisObject F)ᵒᵖ) →
          Functor.obj (Functor.op (incl F) ⋙ coyoneda) x ⟶
            Functor.obj (Functor.obj (const (PointedGaloisObject F)ᵒᵖ) (F ⋙ FintypeCat.incl)) x)
        Opposite.op { obj := A, pt := a, isGalois := isGalois : PointedGaloisObject F } with
      | Opposite.op { obj := A, pt := a, isGalois := isGalois } =>
        {
          app := fun (X : C) ↦
            ↾fun (f : A ⟶ X) ↦
              (@ConcreteCategory.hom _ (ObjectProperty.FullSubcategory.category Finite)
                    (fun (X Y : ObjectProperty.FullSubcategory Finite) ↦ TypeCat.Fun X.obj Y.obj)
                    (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj)
                    (fun (X Y : ObjectProperty.FullSubcategory Finite) ↦
                      (TypeCat.instFunLikeFun : FunLike (TypeCat.Fun X.obj Y.obj) X.obj Y.obj))
                    _ _ _ (F.map f) :
                  (a : (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A)) →
                    (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj X))
                a,
          naturality := ⋯ })
      (map (Functor.obj (const (PointedGaloisObject F)ᵒᵖ) (F ⋙ FintypeCat.incl))
        (Opposite.op { val := f, comp := hf })))
have w : ∀ (Y : C) (g : A ⟶ Y),
  Eq (α :=
    (Functor.obj (Functor.obj (const (PointedGaloisObject F)ᵒᵖ) (F ⋙ FintypeCat.incl))
          (Opposite.op { obj := B, pt := b, isGalois := isGalois_1 : PointedGaloisObject F })).obj
      Y)
    (TypeCat.Fun.toFun
      (ConcreteCategory.hom (FC := TypeCat.Fun)
        (NatTrans.app
          (map (Functor.op (incl F) ⋙ coyoneda)
              (Opposite.op
                { val := f, comp := hf :
                  (Opposite.unop
                        (Opposite.op { obj := B, pt := b, isGalois := isGalois_1 : PointedGaloisObject F })).Hom
                    (Opposite.unop
                      (Opposite.op { obj := A, pt := a, isGalois := isGalois : PointedGaloisObject F })) }) ≫
            match (motive :=
              (x : (PointedGaloisObject F)ᵒᵖ) →
                Functor.obj (Functor.op (incl F) ⋙ coyoneda) x ⟶
                  Functor.obj (Functor.obj (const (PointedGaloisObject F)ᵒᵖ) (F ⋙ FintypeCat.incl)) x)
              Opposite.op { obj := B, pt := b, isGalois := isGalois_1 : PointedGaloisObject F } with
            | Opposite.op { obj := A, pt := a, isGalois := isGalois } =>
              {
                app := fun (X : C) ↦
                  ↾fun (f : A ⟶ X) ↦
                    (@ConcreteCategory.hom _ (ObjectProperty.FullSubcategory.category Finite)
                          (fun (X Y : ObjectProperty.FullSubcategory Finite) ↦ TypeCat.Fun X.obj Y.obj)
                          (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj)
                          (fun (X Y : ObjectProperty.FullSubcategory Finite) ↦
                            (TypeCat.instFunLikeFun : FunLike (TypeCat.Fun X.obj Y.obj) X.obj Y.obj))
                          _ _ _ (F.map f) :
                        (a : (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A)) →
                          (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj X))
                      a,
                naturality := sorry :
                NatTrans
                  (Functor.obj (Functor.op (incl F) ⋙ coyoneda)
                    (Opposite.op { obj := A, pt := a, isGalois := isGalois : PointedGaloisObject F }))
                  (Functor.obj (Functor.obj (const (PointedGaloisObject F)ᵒᵖ) (F ⋙ FintypeCat.incl))
                    (Opposite.op { obj := A, pt := a, isGalois := isGalois : PointedGaloisObject F })) })
          Y))
      g)
    (TypeCat.Fun.toFun
      (ConcreteCategory.hom (FC := TypeCat.Fun)
        (NatTrans.app
          (CategoryStruct.comp (Y :=
            Functor.obj (Functor.obj (const (PointedGaloisObject F)ᵒᵖ) (F ⋙ FintypeCat.incl))
              (Opposite.op { obj := A, pt := a, isGalois := isGalois : PointedGaloisObject F }))
            (match (motive :=
              (x : (PointedGaloisObject F)ᵒᵖ) →
                Functor.obj (Functor.op (incl F) ⋙ coyoneda) x ⟶
                  Functor.obj (Functor.obj (const (PointedGaloisObject F)ᵒᵖ) (F ⋙ FintypeCat.incl)) x)
              Opposite.op { obj := A, pt := a, isGalois := isGalois : PointedGaloisObject F } with
            | Opposite.op { obj := A, pt := a, isGalois := isGalois } =>
              {
                app := fun (X : C) ↦
                  ↾fun (f : A ⟶ X) ↦
                    (@ConcreteCategory.hom _ (ObjectProperty.FullSubcategory.category Finite)
                          (fun (X Y : ObjectProperty.FullSubcategory Finite) ↦ TypeCat.Fun X.obj Y.obj)
                          (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj)
                          (fun (X Y : ObjectProperty.FullSubcategory Finite) ↦
                            (TypeCat.instFunLikeFun : FunLike (TypeCat.Fun X.obj Y.obj) X.obj Y.obj))
                          _ _ _ (F.map f) :
                        (a : (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A)) →
                          (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj X))
                      a,
                naturality := sorry :
                NatTrans
                  (Functor.obj (Functor.op (incl F) ⋙ coyoneda)
                    (Opposite.op { obj := A, pt := a, isGalois := isGalois : PointedGaloisObject F }))
                  (Functor.obj (Functor.obj (const (PointedGaloisObject F)ᵒᵖ) (F ⋙ FintypeCat.incl))
                    (Opposite.op { obj := A, pt := a, isGalois := isGalois : PointedGaloisObject F })) })
            (map (Functor.obj (const (PointedGaloisObject F)ᵒᵖ) (F ⋙ FintypeCat.incl))
              (Opposite.op
                { val := f, comp := hf :
                  (Opposite.unop
                        (Opposite.op { obj := B, pt := b, isGalois := isGalois_1 : PointedGaloisObject F })).Hom
                    (Opposite.unop
                      (Opposite.op { obj := A, pt := a, isGalois := isGalois : PointedGaloisObject F })) })))
          Y))
      g)
Tactic: ext Y (g : (A : C) ⟶ Y)