-- Lean 4 proof step from Mathlib.GroupTheory.GroupAction.MultipleTransitivity (goal)
Goal:
G : Type u_1
inst : Group G
α : Type u_2
inst_1 : MulAction G α
inst_2 : Finite α
k : ℕ
Hk : IsMultiplyPretransitive G α k
s : Set α
hs : s.ncard = k
zero :
  ∀ {G : Type u_1} [inst : Group G] {α : Type u_2} [inst_3 : MulAction G α] [Finite α]
    (Hk : IsMultiplyPretransitive G α 0) {s : Set α} (hs : s.ncard = 0),
    Subgroup.index (fixingSubgroup G s) * (Nat.card α - 0)! = (Nat.card α)!
⊢ Subgroup.index (fixingSubgroup G s) * (Nat.card α - k)! = (Nat.card α)!
have succ : ∀ (k : ℕ)
  (hrec :
    ∀ {G : Type u_1} [inst : Group G] {α : Type u_2} [inst_3 : MulAction G α] [Finite α]
      (Hk : IsMultiplyPretransitive G α k) {s : Set α} (hs : s.ncard = k),
      Subgroup.index (fixingSubgroup G s) * (Nat.card α - k)! = (Nat.card α)!)
  {G : Type u_1} [inst : Group G] {α : Type u_2} [inst_3 : MulAction G α] [Finite α]
  (Hk : IsMultiplyPretransitive G α (k + 1)) {s : Set α} (hs : s.ncard = k + 1),
  Subgroup.index (fixingSubgroup G s) * (Nat.card α - (k + 1))! = (Nat.card α)!
Tactic: induction k generalizing G α with
  | zero =>
    rw [Set.ncard_eq_zero] at hs
    simp [hs]
  | succ k hrec =>
    have hGX : IsPretransitive G α := by
      rw [← is_one_pretransitive_iff]
      apply isMultiplyPretransitive_of_le (n := k + 1)
      · rw [Nat.succ_le_succ_iff]; apply Nat.zero_le
      · rw [← hs, ← Set.ncard_univ]
        exact ncard_le_ncard s.subset_univ finite_univ
    have : s.Nonempty := by
      rw [← Set.ncard_pos, hs]
      exact succ_pos k
    obtain ⟨a, has⟩ := this
    let t : Set (SubMulAction.ofStabilizer G a) := Subtype.val ⁻¹' s
    have hat : Subtype.val '' t = s \ {a} := by
      rw [Set.image_preimage_eq_inter_range]
      simp only [Subtype.range_coe_subtype]
      rw [Set.diff_eq_compl_inter, Set.inter_comm]
      congr
    have hat' : s = insert a (Subtype.val '' t) := by
      rw [hat, Set.insert_diff_singleton, Set.insert_eq_of_mem has]
    have hfs := SubMulAction.fixingSubgroup_of_insert a t
    rw [← hat'] at hfs
    rw [hfs, Subgroup.index_map,
      (MonoidHom.ker_eq_bot_iff (stabilizer G a).subtype).mpr
        (by simp only [Subgroup.coe_subtype, Subtype.coe_injective])]
    simp only [sup_bot_eq, Subgroup.range_subtype]
    have htcard : t.ncard = k := by
      rw [← Nat.succ_inj, Nat.succ_eq_add_one, Nat.succ_eq_add_one, ← hs, hat', eq_comm]
      suffices ¬ a ∈ (Subtype.val '' t) by
        convert! Set.ncard_insert_of_notMem this ?_
        · rw [Set.ncard_image_of_injective _ Subtype.coe_injective]
        apply Set.toFinite
      intro h
      obtain ⟨⟨b, hb⟩, _, hb'⟩ := h
      apply hb
      simp only [← hb', Set.mem_singleton_iff]
    suffices (fixingSubgroup (stabilizer G a) t).index *
      (Nat.card α - 1 - k).factorial =
        (Nat.card α - 1).factorial by
      rw [add_comm k, Nat.mul_right_comm, ← Nat.sub_sub, this, mul_comm,
        index_stabilizer_of_transitive G a]
      exact Nat.mul_factorial_pred (card_ne_zero.mpr ⟨⟨a⟩, inferInstance⟩)
    convert! hrec (ofStabilizer.isMultiplyPretransitive.mp Hk) htcard
    all_goals { rw [nat_card_ofStabilizer_eq G a] }
Cleared hypotheses: G, inst, α, inst_1, inst_2, k, Hk, s, hs