-- Lean 4 proof step from Mathlib.CategoryTheory.Galois.EssSurj (goal)
Goal:
C : Type u₁
inst : Category.{u₂, u₁} C
F : Functor.{u₂, u₁, u₁, u₁ + 1} C FintypeCat
inst_1 : GaloisCategory.{u₁, u₂} C
inst_2 : FiberFunctor F
G : Type u_1
inst_3 : Group G
inst_4 : TopologicalSpace G
inst_5 : IsTopologicalGroup G
inst_6 : CompactSpace G
V U : OpenSubgroup (Aut F)
h : (↑U : Subgroup (Aut F)).Normal
A : C
u : (functorToAction F).obj A ≅ ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ (↑U : Subgroup (Aut F))))
hUinV : U ≤ V
s : Cocone (quotientDiag V h u ⋙ functorToAction F)
g σ : Aut F
⊢ Eq (α := s.pt.V.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))
            _ _ _
            (CategoryStruct.comp (Y :=
                (Cocone.pt (F := quotientDiag V h u ⋙ functorToAction F) (coconeQuotientDiag h u hUinV)).V)
                (((Cocone.pt (F := quotientDiag V h u ⋙ functorToAction F) (coconeQuotientDiag h u hUinV)).ρ :
                    (a : Aut F) → End (coconeQuotientDiag h u hUinV).pt.V)
                  g)
                (FintypeCat.homMk
                  (Quotient.lift
                    (fun (σ : Aut F) ↦
                      (@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))
                            _ _ _
                            (Action.Hom.hom
                                (u.inv ≫
                                  s.ι.app
                                    (SingleObj.star
                                      (HasQuotient.Quotient ↥(↑V : Subgroup (Aut F)) (B :=
                                        Subgroup ↥(↑V : Subgroup (Aut F)))
                                        ((↑U : Subgroup (Aut F)).subgroupOf (↑V : Subgroup (Aut F)))))) :
                              Action.V (G := Aut F)
                                  (ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ (↑U : Subgroup (Aut F))))) ⟶
                                s.pt.V) :
                          (a : (ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ (↑U : Subgroup (Aut F))))).V.obj) →
                            s.pt.V.obj)
                        ⟦σ⟧)
                    ⋯)) :
              (Cocone.pt (F := quotientDiag V h u ⋙ functorToAction F) (coconeQuotientDiag h u hUinV)).V ⟶ s.pt.V) :
          (a : (coconeQuotientDiag h u hUinV).pt.V.obj) → s.pt.V.obj)
        ⟦σ⟧ :
      s.pt.V.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))
            _ _ _
            (CategoryStruct.comp (Y := s.pt.V)
                (FintypeCat.homMk
                  (Quotient.lift
                    (fun (σ : Aut F) ↦
                      (@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))
                            _ _ _
                            (Action.Hom.hom
                                (u.inv ≫
                                  s.ι.app
                                    (SingleObj.star
                                      (HasQuotient.Quotient ↥(↑V : Subgroup (Aut F)) (B :=
                                        Subgroup ↥(↑V : Subgroup (Aut F)))
                                        ((↑U : Subgroup (Aut F)).subgroupOf (↑V : Subgroup (Aut F)))))) :
                              Action.V (G := Aut F)
                                  (ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ (↑U : Subgroup (Aut F))))) ⟶
                                s.pt.V) :
                          (a : (ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ (↑U : Subgroup (Aut F))))).V.obj) →
                            s.pt.V.obj)
                        ⟦σ⟧)
                    ⋯))
                ((s.pt.ρ : (a : Aut F) → End s.pt.V) g) :
              (Cocone.pt (F := quotientDiag V h u ⋙ functorToAction F) (coconeQuotientDiag h u hUinV)).V ⟶ s.pt.V) :
          (a : (coconeQuotientDiag h u hUinV).pt.V.obj) → s.pt.V.obj)
        ⟦σ⟧ :
      s.pt.V.obj)
have h₁ : Eq (α :=
  (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj)
    (obj (C :=
        SingleObj.{u₁}
          (HasQuotient.Quotient ↥(↑V : Subgroup (Aut F)) (B := Subgroup ↥(↑V : Subgroup (Aut F)))
            ((↑U : Subgroup (Aut F)).subgroupOf (↑V : Subgroup (Aut F)))))
        (obj
          (const
            (SingleObj.{u₁}
              (HasQuotient.Quotient ↥(↑V : Subgroup (Aut F)) (B := Subgroup ↥(↑V : Subgroup (Aut F)))
                ((↑U : Subgroup (Aut F)).subgroupOf (↑V : Subgroup (Aut F))))))
          s.pt)
        (SingleObj.star
          (HasQuotient.Quotient ↥(↑V : Subgroup (Aut F)) (B := Subgroup ↥(↑V : Subgroup (Aut F)))
            ((↑U : Subgroup (Aut F)).subgroupOf (↑V : Subgroup (Aut F)))))).V)
  ((@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))
          _ _ _
          (((Action.ρ (ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ (↑U : Subgroup (Aut F))))) :
                    (a : Aut F) → End (ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ (↑U : Subgroup (Aut F))))).V)
                  g ≫
                u.inv.hom) ≫
              Action.Hom.hom
                (s.ι.app
                  (SingleObj.star
                    (HasQuotient.Quotient ↥(↑V : Subgroup (Aut F)) (B := Subgroup ↥(↑V : Subgroup (Aut F)))
                      ((↑U : Subgroup (Aut F)).subgroupOf (↑V : Subgroup (Aut F)))))) :
            Action.V (G := Aut F) (ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ (↑U : Subgroup (Aut F))))) ⟶
              (obj (C :=
                  SingleObj.{u₁}
                    (HasQuotient.Quotient ↥(↑V : Subgroup (Aut F)) (B := Subgroup ↥(↑V : Subgroup (Aut F)))
                      ((↑U : Subgroup (Aut F)).subgroupOf (↑V : Subgroup (Aut F)))))
                  (obj
                    (const
                      (SingleObj.{u₁}
                        (HasQuotient.Quotient ↥(↑V : Subgroup (Aut F)) (B := Subgroup ↥(↑V : Subgroup (Aut F)))
                          ((↑U : Subgroup (Aut F)).subgroupOf (↑V : Subgroup (Aut F))))))
                    s.pt)
                  (SingleObj.star
                    (HasQuotient.Quotient ↥(↑V : Subgroup (Aut F)) (B := Subgroup ↥(↑V : Subgroup (Aut F)))
                      ((↑U : Subgroup (Aut F)).subgroupOf (↑V : Subgroup (Aut F)))))).V) :
        (a :
            (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj)
              (ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ (↑U : Subgroup (Aut F))))).V) →
          (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj)
            (((const
                        (SingleObj
                          (↥(↑V : Subgroup (Aut F)) ⧸ (↑U : Subgroup (Aut F)).subgroupOf (↑V : Subgroup (Aut F))))).obj
                    s.pt).obj
                (SingleObj.star
                  (↥(↑V : Subgroup (Aut F)) ⧸ (↑U : Subgroup (Aut F)).subgroupOf (↑V : Subgroup (Aut F))))).V)
      ⟦σ⟧ :
    (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj)
      (obj (C :=
          SingleObj.{u₁}
            (HasQuotient.Quotient ↥(↑V : Subgroup (Aut F)) (B := Subgroup ↥(↑V : Subgroup (Aut F)))
              ((↑U : Subgroup (Aut F)).subgroupOf (↑V : Subgroup (Aut F)))))
          (obj
            (const
              (SingleObj.{u₁}
                (HasQuotient.Quotient ↥(↑V : Subgroup (Aut F)) (B := Subgroup ↥(↑V : Subgroup (Aut F)))
                  ((↑U : Subgroup (Aut F)).subgroupOf (↑V : Subgroup (Aut F))))))
            s.pt)
          (SingleObj.star
            (HasQuotient.Quotient ↥(↑V : Subgroup (Aut F)) (B := Subgroup ↥(↑V : Subgroup (Aut F)))
              ((↑U : Subgroup (Aut F)).subgroupOf (↑V : Subgroup (Aut F)))))).V)
  ((@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))
          _ _ _
          (Action.Hom.hom
                (s.ι.app
                  (SingleObj.star
                    (HasQuotient.Quotient ↥(↑V : Subgroup (Aut F)) (B := Subgroup ↥(↑V : Subgroup (Aut F)))
                      ((↑U : Subgroup (Aut F)).subgroupOf (↑V : Subgroup (Aut F)))))) ≫
              (s.pt.ρ : (a : Aut F) → End s.pt.V) g :
            ((quotientDiag V h u ⋙ functorToAction F).obj
                  (SingleObj.star
                    (HasQuotient.Quotient ↥(↑V : Subgroup (Aut F)) (B := Subgroup ↥(↑V : Subgroup (Aut F)))
                      ((↑U : Subgroup (Aut F)).subgroupOf (↑V : Subgroup (Aut F)))))).V ⟶
              s.pt.V) :
        (a :
            (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj)
              ((quotientDiag V h u ⋙ functorToAction F).obj
                  (SingleObj.star
                    (↥(↑V : Subgroup (Aut F)) ⧸ (↑U : Subgroup (Aut F)).subgroupOf (↑V : Subgroup (Aut F))))).V) →
          (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) s.pt.V)
      ((@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))
              _ _ _ u.inv.hom :
            (a :
                (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj)
                  (ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ (↑U : Subgroup (Aut F))))).V) →
              (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) ((functorToAction F).obj A).V)
          ⟦σ⟧ :
        (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) ((functorToAction F).obj A).V) :
    (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) s.pt.V)
Tactic: change (((Aut F ⧸ₐ U.toSubgroup).ρ g ≫ u.inv.hom) ≫ (s.ι.app (SingleObj.star _)).hom) ⟦σ⟧ =
      ((s.ι.app (SingleObj.star _)).hom ≫ s.pt.ρ g) (u.inv.hom ⟦σ⟧)