-- Lean 4 proof step from Mathlib.GroupTheory.FreeGroup.NielsenSchreier (goal)
Goal:
G A : Type u
inst : Group G
inst_1 : IsFreeGroup G
inst_2 : MulAction G A
X : Type u
inst_3 : Group X
f : Labelling (Generators (ActionCategory G A)) X
f' : IsFreeGroup.Generators G → (A → X) ⋊[mulAutArrow] G :=
  fun (e : IsFreeGroup.Generators G) ↦
    ⟨fun (b : A) ↦
      f (a :=
        Sigma.mk (β := fun (c : CategoryTheory.SingleObj G) ↦ Functor.obj (actionAsFunctor G A) c) ()
          ((IsFreeGroup.of e)⁻¹ • b))
        (b := Sigma.mk (β := fun (c : CategoryTheory.SingleObj G) ↦ Functor.obj (actionAsFunctor G A) c) () b)
        (Subtype.mk (p := fun (e_1 : IsFreeGroup.Generators G) ↦
          HSMul.hSMul (β := A) (IsFreeGroup.of e_1)
              (ActionCategory.back (M := G)
                (Sigma.mk (β := fun (c : CategoryTheory.SingleObj G) ↦ Functor.obj (actionAsFunctor G A) c) ()
                  ((IsFreeGroup.of e)⁻¹ • b))) =
            ActionCategory.back (M := G)
              (Sigma.mk (β := fun (c : CategoryTheory.SingleObj G) ↦ Functor.obj (actionAsFunctor G A) c) () b))
          e ⋯),
      IsFreeGroup.of e⟩
⊢ ∃! F : ActionCategory G A ⥤ CategoryTheory.SingleObj X,
    ∀ (a b : Generators (ActionCategory G A)) (g : a ⟶ b),
      F.map
          (Subtype.mk (p :=
            fun
              (f :
                Sigma.fst (β := fun (c : CategoryTheory.SingleObj G) ↦ Functor.obj (actionAsFunctor G A) c)
                    (have this := a;
                    this) ⟶
                  b.fst (β := fun (c : CategoryTheory.SingleObj G) ↦ Functor.obj (actionAsFunctor G A) c)) ↦
            (ConcreteCategory.hom (FC := TypeCat.Fun) (CategoryTheory.Functor.map (actionAsFunctor G A) f) :
                  (a :
                      (fun (X : Type u) ↦ X)
                        ((actionAsFunctor G A).obj
                          (have this := a;
                            this).fst)) →
                    (fun (X : Type u) ↦ X) ((actionAsFunctor G A).obj b.fst))
                (Sigma.snd (β := fun (c : CategoryTheory.SingleObj G) ↦ Functor.obj (actionAsFunctor G A) c)
                  (have this := a;
                  this)) =
              b.snd (β := fun (c : CategoryTheory.SingleObj G) ↦ Functor.obj (actionAsFunctor G A) c))
            (IsFreeGroup.of (↑g : IsFreeGroup.Generators G)) ⋯) =
        f g
have rcases : ∀ (F' : G →* (A → X) ⋊[mulAutArrow] G)
  (hF' : ∀ (a : IsFreeGroup.Generators G), (F' : (a : G) → (A → X) ⋊[mulAutArrow] G) (IsFreeGroup.of a) = f' a)
  (uF' :
    ∀ (y : G →* (A → X) ⋊[mulAutArrow] G),
      (fun (F : G →* (A → X) ⋊[mulAutArrow] G) ↦
            ∀ (a : IsFreeGroup.Generators G), (F : (a : G) → (A → X) ⋊[mulAutArrow] G) (IsFreeGroup.of a) = f' a)
          y →
        y = F'),
  ∃! F : ActionCategory G A ⥤ CategoryTheory.SingleObj X,
    ∀ (a b : Generators (ActionCategory G A)) (g : a ⟶ b),
      F.map
          (Subtype.mk (p :=
            fun
              (f :
                Sigma.fst (β := fun (c : CategoryTheory.SingleObj G) ↦ Functor.obj (actionAsFunctor G A) c)
                    (have this := a;
                    this) ⟶
                  b.fst (β := fun (c : CategoryTheory.SingleObj G) ↦ Functor.obj (actionAsFunctor G A) c)) ↦
            (ConcreteCategory.hom (FC := TypeCat.Fun) (CategoryTheory.Functor.map (actionAsFunctor G A) f) :
                  (a :
                      (fun (X : Type u) ↦ X)
                        ((actionAsFunctor G A).obj
                          (have this := a;
                            this).fst)) →
                    (fun (X : Type u) ↦ X) ((actionAsFunctor G A).obj b.fst))
                (Sigma.snd (β := fun (c : CategoryTheory.SingleObj G) ↦ Functor.obj (actionAsFunctor G A) c)
                  (have this := a;
                  this)) =
              b.snd (β := fun (c : CategoryTheory.SingleObj G) ↦ Functor.obj (actionAsFunctor G A) c))
            (IsFreeGroup.of (↑g : IsFreeGroup.Generators G)) sorry) =
        f g
Tactic: rcases IsFreeGroup.unique_lift f' with ⟨F', hF', uF'⟩