-- 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)
σ τ : Aut F
hst : σ ≈ τ
J' : SingleObj.{u₁}
    (HasQuotient.Quotient ↥(↑V : Subgroup (Aut F)) (B := Subgroup ↥(↑V : Subgroup (Aut F)))
      ((↑U : Subgroup (Aut F)).subgroupOf (↑V : Subgroup (Aut F)))) ⥤
  Action FintypeCat (Aut F) :=
  quotientDiag V h u ⋙ functorToAction F
m : End (C :=
  SingleObj.{u₁}
    (HasQuotient.Quotient ↥(↑V : Subgroup (Aut F)) (B := Subgroup ↥(↑V : Subgroup (Aut F)))
      ((↑U : Subgroup (Aut F)).subgroupOf (↑V : Subgroup (Aut F)))))
  (SingleObj.star
    (HasQuotient.Quotient ↥(↑V : Subgroup (Aut F)) (B := Subgroup ↥(↑V : Subgroup (Aut F)))
      ((↑U : Subgroup (Aut F)).subgroupOf (↑V : Subgroup (Aut F))))) :=
  ⟦Subtype.mk (p := fun (x : Aut F) ↦ x ∈ (↑V : Subgroup (Aut F))) (σ⁻¹ * τ) ⋯⟧
h1 :
  J'.map m ≫
      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.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)))))
⊢ Eq.{u₁ + 1} (α := s.pt.V.obj)
    ((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))))) ⟶
                    (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))
      σ)
    ((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))))) ⟶
                    (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))
      τ)
have conv_rhs : Eq.{u₁ + 1} (α := s.pt.V.obj)
  ((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))))) ⟶
                  (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))
    σ)
  ((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))
              _ _ _
              (u.inv ≫
                  J'.map m ≫
                    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)))))).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)
                (((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))
    τ)
Tactic: conv_rhs => rw [← h1]