-- Lean 4 proof step from Mathlib.CategoryTheory.Galois.IsFundamentalgroup (hypothesis)
Goal:
C : Type u₁
inst : Category.{u₂, u₁} C
F : Functor.{u₂, w, u₁, w + 1} C FintypeCat
G : Type u_1
inst_1 : Group G
inst_2 : (X : C) → MulAction.{u_1, w} G (F.obj X).obj
inst_3 : IsNaturalSMul F G
inst_4 : GaloisCategory.{u₁, u₂} C
inst_5 : FiberFunctor F
t : F ⟶ F
X : C
inst_6 : IsGalois.{u₁, u₂} X
g : G
y : (F.obj X).obj
φ : Aut X
hg :
  HSMul.hSMul (β := (F.obj X).obj) g
      ((@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 φ.hom) :
            (a : (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj X)) →
              (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj X))
          y :
        (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj X)) =
    (@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))
          _ _ _ (t.app X) :
        (a : (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj X)) →
          (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj X))
      ((@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 φ.hom) :
            (a : (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj X)) →
              (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj X))
          y :
        (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj X))
⊢ g • y =
    (@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))
          _ _ _ (t.app X) :
        (a : (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj X)) →
          (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj X))
      y
have this : Function.Injective (α := (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj X)) (β :=
  (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj X))
  (⇑(@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 φ.hom)) :
    (a : (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj X)) →
      (fun (X : ObjectProperty.FullSubcategory Finite) ↦ X.obj) (F.obj X))
Tactic: have : Function.Injective (F.map φ.hom) :=
    ConcreteCategory.injective_of_mono_of_preservesPullback (F.map φ.hom)