-- Lean 4 proof step from Mathlib.CategoryTheory.Action.Concrete (goal)
Goal:
G : Type u_1
inst : Group G
H N : Subgroup G
inst_1 : Fintype (G ⧸ N)
inst_2 : N.Normal
σ τ : G
⊢ Hom.hom { hom := FintypeCat.homMk (Quotient.lift (fun (σ_1 : G) ↦ ⟦σ_1 * (σ * τ)⁻¹⟧) ⋯), comm := ⋯ } =
    (HMul.hMul (α := End (C := Action FintypeCat G) (ofMulAction G (FintypeCat.of (G ⧸ N))))
        { hom := FintypeCat.homMk (Quotient.lift (fun (σ_1 : G) ↦ ⟦σ_1 * σ⁻¹⟧) ⋯), comm := ⋯ :
          Hom (ofMulAction G (FintypeCat.of (G ⧸ N))) (ofMulAction G (FintypeCat.of (G ⧸ N))) }
        { hom := FintypeCat.homMk (Quotient.lift (fun (σ : G) ↦ ⟦σ * τ⁻¹⟧) ⋯), comm := ⋯ :
          Hom (ofMulAction G (FintypeCat.of (G ⧸ N))) (ofMulAction G (FintypeCat.of (G ⧸ N))) }).hom
have h : ∀ (x : G ⧸ N),
  Eq (α := (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (V (ofMulAction G (FintypeCat.of (G ⧸ N)))))
    ((@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))
            _ _ _
            (Hom.hom { hom := FintypeCat.homMk (Quotient.lift (fun (σ_1 : G) ↦ ⟦σ_1 * (σ * τ)⁻¹⟧) sorry), comm := sorry } :
              V (ofMulAction G (FintypeCat.of (G ⧸ N))) ⟶ V (ofMulAction G (FintypeCat.of (G ⧸ N)))) :
          (a : (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (ofMulAction G (FintypeCat.of (G ⧸ N))).V) →
            (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (ofMulAction G (FintypeCat.of (G ⧸ N))).V)
        x :
      (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (V (ofMulAction G (FintypeCat.of (G ⧸ N)))))
    ((@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))
            _ _ _
            (HMul.hMul (α := End (C := Action FintypeCat G) (ofMulAction G (FintypeCat.of (G ⧸ N))))
                { hom := FintypeCat.homMk (Quotient.lift (fun (σ_1 : G) ↦ ⟦σ_1 * σ⁻¹⟧) sorry), comm := sorry :
                  Hom (ofMulAction G (FintypeCat.of (G ⧸ N))) (ofMulAction G (FintypeCat.of (G ⧸ N))) }
                { hom := FintypeCat.homMk (Quotient.lift (fun (σ : G) ↦ ⟦σ * τ⁻¹⟧) sorry), comm := sorry :
                  Hom (ofMulAction G (FintypeCat.of (G ⧸ N))) (ofMulAction G (FintypeCat.of (G ⧸ N))) }).hom :
          (a : (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (ofMulAction G (FintypeCat.of (G ⧸ N))).V) →
            (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (ofMulAction G (FintypeCat.of (G ⧸ N))).V)
        x :
      (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (V (ofMulAction G (FintypeCat.of (G ⧸ N)))))
Tactic: ext (x : G ⧸ N)