-- 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
m :
  HasQuotient.Quotient ↥V (B := Subgroup ↥(↑V : Subgroup (Aut F)))
    ((↑U : Subgroup (Aut F)).subgroupOf (↑V : Subgroup (Aut F)))
⊢ Eq (α :=
    (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))))) ⟶
      ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ (↑V : Subgroup (Aut F)))))
    (CategoryStruct.comp (Y :=
        (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))))))
        ((functorToAction F).map ((quotientDiag V h u).map m))
        (u.hom ≫ quotientToQuotientOfLE (↑V : Subgroup (Aut F)) (↑U : Subgroup (Aut F)) hUinV) :
      (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))))) ⟶
        ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ (↑V : Subgroup (Aut F)))))
    (u.hom ≫ quotientToQuotientOfLE (↑V : Subgroup (Aut F)) (↑U : Subgroup (Aut F)) hUinV :
      (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))))) ⟶
        ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ (↑V : Subgroup (Aut F)))))
have rw : Eq (α :=
  Quiver.Hom (V := Action FintypeCat (Aut F)) (ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ (↑U : Subgroup (Aut F)))))
    (ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ (↑V : Subgroup (Aut F))))))
  (u.inv ≫
      CategoryStruct.comp (Y :=
        (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))))))
        ((functorToAction F).map ((quotientDiag V h u).map m))
        (u.hom ≫ quotientToQuotientOfLE (↑V : Subgroup (Aut F)) (↑U : Subgroup (Aut F)) hUinV) :
    Quiver.Hom (V := Action FintypeCat (Aut F)) (ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ (↑U : Subgroup (Aut F)))))
      (ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ (↑V : Subgroup (Aut F))))))
  (quotientToQuotientOfLE (↑V : Subgroup (Aut F)) (↑U : Subgroup (Aut F)) hUinV :
    Quiver.Hom (V := Action FintypeCat (Aut F)) (ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ (↑U : Subgroup (Aut F)))))
      (ofMulAction (Aut F) (FintypeCat.of (Aut F ⧸ (↑V : Subgroup (Aut F))))))
Tactic: rw [← cancel_epi (u.inv), Iso.inv_hom_id_assoc]