-- Lean 4 proof step from Mathlib.GroupTheory.GroupAction.Jordan (goal)
Goal:
G : Type u_1
α : Type u_2
inst : Group G
inst_1 : MulAction G α
hG : IsPreprimitive G α
s : Set α
n : ℕ
hsn : s.ncard = n + 1
hsn' : n + 2 < Nat.card α
⊢ (IsPretransitive.{u_1, u_2} ↥(fixingSubgroup G s) ↥(ofFixingSubgroup G s) → IsMultiplyPretransitive G α 2) ∧
    (IsPreprimitive.{u_1, u_2} ↥(fixingSubgroup G s) ↥(ofFixingSubgroup G s) → IsMultiplyPreprimitive G α 2)
have h : ∀ (n : ℕ)
  (hrec :
    ∀ m < n,
      ∀ {G : Type u_1} {α : Type u_2} [inst : Group G] [inst_2 : MulAction G α] (hG : IsPreprimitive G α) {s : Set α}
        (hsn : s.ncard = m + 1) (hsn' : m + 2 < Nat.card α),
        (IsPretransitive.{u_1, u_2} ↥(fixingSubgroup G s) ↥(ofFixingSubgroup G s) → IsMultiplyPretransitive G α 2) ∧
          (IsPreprimitive.{u_1, u_2} ↥(fixingSubgroup G s) ↥(ofFixingSubgroup G s) → IsMultiplyPreprimitive G α 2))
  {G : Type u_1} {α : Type u_2} [inst : Group G] [inst_2 : MulAction G α] (hG : IsPreprimitive G α) {s : Set α}
  (hsn : s.ncard = n + 1) (hsn' : n + 2 < Nat.card α),
  (IsPretransitive.{u_1, u_2} ↥(fixingSubgroup G s) ↥(ofFixingSubgroup G s) → IsMultiplyPretransitive G α 2) ∧
    (IsPreprimitive.{u_1, u_2} ↥(fixingSubgroup G s) ↥(ofFixingSubgroup G s) → IsMultiplyPreprimitive G α 2)
Tactic: induction n using Nat.strong_induction_on generalizing α G with
  | h n hrec =>
  have : Finite α := Nat.finite_of_card_ne_zero <| ne_zero_of_lt hsn'
  have hs_ne_univ : s ≠ Set.univ := by
    intro hs
    rw [hs, Set.ncard_univ] at hsn
    simp only [hsn, add_lt_add_iff_left, Nat.not_ofNat_lt_one] at hsn'
  have hs_nonempty : s.Nonempty := by
    simp [← Set.ncard_pos s.toFinite, hsn]
  -- The result is assumed by induction for sets of ncard ≤ n
  rcases Nat.lt_or_ge (n + 1) 2 with hn | hn
  · -- When n + 1 < 2 (imposes n = 0)
    have hn : n = 0 := by
      rwa [Nat.succ_lt_succ_iff, Nat.lt_one_iff] at hn
    simp only [hn, zero_add, Set.ncard_eq_one] at hsn
    obtain ⟨a, hsa⟩ := hsn
    suffices IsPretransitive (fixingSubgroup G s) (ofFixingSubgroup G s) →
      IsMultiplyPretransitive G α 2 by
      refine ⟨this, fun hs_prim ↦ ?_⟩
      rw [hsa] at hs_prim
      rw [isMultiplyPreprimitive_succ_iff_ofStabilizer G α le_rfl (a := a),
        is_one_preprimitive_iff]
      exact IsPreprimitive.of_surjective
          ofFixingSubgroup_of_singleton_bijective.surjective
    rw [hsa]
    rw [ofStabilizer.isMultiplyPretransitive (a := a)]
    rw [is_one_pretransitive_iff]
    exact IsPretransitive.of_surjective_map
      ofFixingSubgroup_of_singleton_bijective.surjective
  rcases Nat.lt_or_ge (2 * (n + 1)) (Nat.card α) with hn1 | hn2
  · -- CASE where 2 * s.ncard < Nat.card α
    -- get a, b ∈ s, a ≠ b
    have : 1 < s.ncard := by rwa [hsn]
    rw [Set.one_lt_ncard] at this
    obtain ⟨a, ha, b, hb, hab⟩ := this
    -- apply Rudio to get g ∈ G such that a ∈ g • s, b ∉ g • s
    obtain ⟨g, hga, hgb⟩ :=
      exists_mem_smul_and_notMem_smul (G := G) s.toFinite hs_nonempty hs_ne_univ hab
    let t := s ∩ g • s
    have ht : t.Finite := s.toFinite.inter_of_left (g • s)
    have htm : t.ncard = t.ncard - 1 + 1 := by
      apply (Nat.sub_eq_iff_eq_add ?_).mp rfl
      rw [Nat.one_le_iff_ne_zero]
      apply Set.ncard_ne_zero_of_mem (a := a) _ ht
      exact ⟨ha, hga⟩
    have hmn : t.ncard - 1 < n := by
      rw [Nat.lt_iff_add_one_le, ← htm, Nat.le_iff_lt_add_one, ← hsn]
      apply Set.ncard_lt_ncard _
      exact ⟨Set.inter_subset_left, fun h ↦ hgb (Set.inter_subset_right (h hb))⟩
    have htm' : t.ncard - 1 + 2 < Nat.card α := lt_trans (Nat.add_lt_add_right hmn 2) hsn'
    suffices IsPretransitive ↥(fixingSubgroup G s) ↥(ofFixingSubgroup G s) →
      IsMultiplyPretransitive G α 2 by
      refine ⟨this, fun hs_prim ↦ ?_⟩
      have ht_prim : IsPreprimitive (fixingSubgroup G t) (ofFixingSubgroup G t) := by
        apply IsPreprimitive.isPreprimitive_ofFixingSubgroup_inter hs_prim
        apply Set.union_ne_univ_of_ncard_add_ncard_lt
        rwa [Set.ncard_smul_set, hsn, ← two_mul]
      apply (hrec (t.ncard - 1) hmn hG htm htm').2 ht_prim
    intro hs_trans
    have ht_trans : IsPretransitive (fixingSubgroup G t) (ofFixingSubgroup G t) :=
      IsPretransitive.isPretransitive_ofFixingSubgroup_inter hs_trans (by
        apply Set.union_ne_univ_of_ncard_add_ncard_lt
        rwa [Set.ncard_smul_set, hsn, ← two_mul])
    apply (hrec (t.ncard - 1) hmn hG htm ?_).1 ht_trans
    apply lt_trans _ hsn'
    exact Nat.add_lt_add_right hmn 2
  · -- CASE : 2 * s.ncard ≥ Nat.card α
    have : Set.Nontrivial sᶜ := by
      rwa [← Set.one_lt_encard_iff_nontrivial, ← sᶜ.toFinite.cast_ncard_eq, Nat.one_lt_cast,
        ← Nat.add_lt_add_iff_left, Set.ncard_add_ncard_compl, add_comm, hsn, add_comm]
    -- get a, b ∈ sᶜ, a ≠ b
    obtain ⟨a, ha : a ∈ sᶜ, b, hb : b ∈ sᶜ, hab⟩ := this
    -- apply Rudio to get g ∈ G such that a ∈ g • sᶜ, b ∉ g • sᶜ
    obtain ⟨g, hga, hgb⟩ := exists_mem_smul_and_notMem_smul (G := G)
      sᶜ.toFinite (Set.nonempty_of_mem ha)
      (by simpa [Set.nonempty_iff_ne_empty] using hs_nonempty)
      hab
    let t := s ∩ g • s
    have ha : a ∉ s ∪ g • s := by
      simp only [Set.smul_set_compl, Set.mem_compl_iff] at hga ha
      simp [ha, hga]
    have htm : t.ncard = t.ncard - 1 + 1 := by
      apply (Nat.sub_eq_iff_eq_add ?_).mp rfl
      rw [Nat.one_le_iff_ne_zero, ← Nat.pos_iff_ne_zero, Set.ncard_pos]
      apply Set.nonempty_inter_of_le_ncard_add_ncard
      · rw [Set.ncard_smul_set, ← two_mul, hsn]; exact hn2
      · exact fun h ↦ ha (by rw [h]; trivial)
    have hmn : t.ncard - 1 < n := by
      rw [Nat.lt_iff_add_one_le, ← htm, Nat.le_iff_lt_add_one, ← hsn]
      apply Set.ncard_lt_ncard _
      refine ⟨Set.inter_subset_left, fun h ↦ hb ?_⟩
      suffices s = g • s by
        rw [this]
        simpa only [Set.smul_set_compl, Set.mem_compl_iff, Set.not_notMem] using hgb
      apply Set.eq_of_subset_of_ncard_le _ _ (g • s).toFinite
      · exact subset_trans h Set.inter_subset_right
      · rw [Set.ncard_smul_set]
    have htm' : t.ncard - 1 + 2 < Nat.card α := lt_trans (Nat.add_lt_add_right hmn 2) hsn'
    have hsgs_ne_top : s ∪ g • s ≠ ⊤ := fun h ↦ ha (h ▸ Set.mem_univ a)
    suffices IsPretransitive ↥(fixingSubgroup G s) ↥(ofFixingSubgroup G s) →
      IsMultiplyPretransitive G α 2 by
      refine ⟨this, fun hs_prim ↦ ?_⟩
      apply (hrec _ hmn hG htm htm').2
      exact IsPreprimitive.isPreprimitive_ofFixingSubgroup_inter
          hs_prim hsgs_ne_top
    intro hs_trans
    apply (hrec _ hmn hG htm htm').1
    exact IsPretransitive.isPretransitive_ofFixingSubgroup_inter hs_trans hsgs_ne_top
Cleared hypotheses: G, α, inst, inst_1, hG, s, n, hsn, hsn'