-- 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
inst_2 : FiberFunctor F
f g : End F
A : PointedGaloisObject F
⊢ Eq (α := (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A.obj))
    ((@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.app A.obj : F.obj A.obj ⟶ F.obj A.obj) :
          (a : (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A.obj)) →
            (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A.obj))
        ((@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))
                _ _ _ (g.app A.obj : F.obj A.obj ⟶ F.obj A.obj) :
              (a : (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A.obj)) →
                (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A.obj))
            A.pt :
          (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A.obj)) :
      (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A.obj))
    ((@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.app A.obj ≫
                F.map
                  (Iso.hom
                    ((AutGalois.π F A : (a : AutGalois F) → Aut A.obj)
                      ((endEquivAutGalois F : (a : End F) → AutGalois F) g))) :
              F.obj A.obj ⟶ F.obj A.obj) :
          (a : (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A.obj)) →
            (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A.obj))
        A.pt :
      (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A.obj))
have rw : Eq (α := (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A.obj))
  ((@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.app A.obj : F.obj A.obj ⟶ F.obj A.obj) :
        (a : (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A.obj)) →
          (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A.obj))
      ((@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))
              _ _ _ (g.app A.obj : F.obj A.obj ⟶ F.obj A.obj) :
            (a : (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A.obj)) →
              (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A.obj))
          A.pt :
        (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A.obj)) :
    (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A.obj))
  ((@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
                (Iso.hom
                  ((AutGalois.π F A : (a : AutGalois F) → Aut A.obj)
                    ((endEquivAutGalois F : (a : End F) → AutGalois F) g))) ≫
              f.app A.obj :
            F.obj A.obj ⟶ F.obj A.obj) :
        (a : (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A.obj)) →
          (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A.obj))
      A.pt :
    (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj A.obj))
Tactic: rw [← f.naturality, FintypeCat.comp_apply, endEquivAutGalois_π]