-- Lean 4 proof step from Mathlib.GroupTheory.Sylow (goal)
Goal:
p : ℕ
G : Type u_1
inst : Group G
hp : Fact (Nat.Prime p)
inst_1 : Finite.{u_1 + 1} (Sylow p G)
P Q : Sylow p G
have have₁ have₂ :
  ∀
    (H :
      ∀ {R : Sylow p G} {S : (↑(orbit G P) : Type u_1)},
        Membership.mem (γ := Set (↑(orbit G P) : Type u_1)) (fixedPoints ↥(↑R : Subgroup G) (↑(orbit G P) : Type u_1))
            S ↔
          (↑(↑S : Sylow p G) : Subgroup G) = (↑R : Subgroup G)),
    ∃ (g : G), g • P = Q
have₃ :
  ∀
    (H :
      ∀ {R : Sylow p G} {S : (↑(orbit G P) : Type u_1)},
        Membership.mem (γ := Set (↑(orbit G P) : Type u_1)) (fixedPoints ↥(↑R : Subgroup G) (↑(orbit G P) : Type u_1))
            S ↔
          (↑(↑S : Sylow p G) : Subgroup G) = (↑R : Subgroup G)),
    Set.Nonempty.{u_1} (α := (↑(orbit G P) : Type u_1)) (fixedPoints ↥(↑Q : Subgroup G) (↑(orbit G P) : Type u_1))
hpα :
  ∀
    (H :
      ∀ {R : Sylow p G} {S : (↑(orbit G P) : Type u_1)},
        Membership.mem (γ := Set (↑(orbit G P) : Type u_1)) (fixedPoints ↥(↑R : Subgroup G) (↑(orbit G P) : Type u_1))
            S ↔
          (↑(↑S : Sylow p G) : Subgroup G) = (↑R : Subgroup G)),
    ¬p ∣ Nat.card.{u_1} (↑(orbit G P) : Type u_1)
hpα₁ :
  ∀
    (H :
      ∀ {R : Sylow p G} {S : (↑(orbit G P) : Type u_1)},
        Membership.mem (γ := Set (↑(orbit G P) : Type u_1)) (fixedPoints ↥(↑R : Subgroup G) (↑(orbit G P) : Type u_1))
            S ↔
          (↑(↑S : Sylow p G) : Subgroup G) = (↑R : Subgroup G))
    (h : p ∣ Nat.card.{u_1} (↑(orbit G P) : Type u_1)), 1 ≡ 0 [MOD p]
hpα₂ :
  ∀
    (H :
      ∀ {R : Sylow p G} {S : (↑(orbit G P) : Type u_1)},
        Membership.mem (γ := Set (↑(orbit G P) : Type u_1)) (fixedPoints ↥(↑R : Subgroup G) (↑(orbit G P) : Type u_1))
            S ↔
          (↑(↑S : Sylow p G) : Subgroup G) = (↑R : Subgroup G))
    (h : p ∣ Nat.card.{u_1} (↑(orbit G P) : Type u_1)),
    1 = Nat.card.{u_1} (↑(fixedPoints ↥(↑P : Subgroup G) (↑(orbit G P) : Type u_1)) : Type u_1)
⊢ ∃ (g : G), g • P = Q
have hpα₃ : ∀
  (H :
    ∀ {R : Sylow p G} {S : (↑(orbit G P) : Type u_1)},
      Membership.mem (γ := Set (↑(orbit G P) : Type u_1)) (fixedPoints ↥(↑R : Subgroup G) (↑(orbit G P) : Type u_1)) S ↔
        (↑(↑S : Sylow p G) : Subgroup G) = (↑R : Subgroup G))
  (h : p ∣ Nat.card.{u_1} (↑(orbit G P) : Type u_1)),
  Nat.card.{u_1} (↑(fixedPoints ↥(↑P : Subgroup G) (↑(orbit G P) : Type u_1)) : Type u_1) =
    Nat.card.{u_1} (↑{Subtype.mk (p := fun (x : Sylow p G) ↦ x ∈ orbit G P) P sorry} : Type u_1)
Tactic: classical
      have H := fun {R : Sylow p G} {S : orbit G P} =>
        calc
          S ∈ fixedPoints R (orbit G P) ↔ S.1 ∈ fixedPoints R (Sylow p G) :=
            forall_congr' fun a => Subtype.ext_iff
          _ ↔ R.1 ≤ S := R.2.sylow_mem_fixedPoints_iff
          _ ↔ S.1.1 = R := ⟨fun h => R.3 S.1.2 h, ge_of_eq⟩
      suffices Set.Nonempty (fixedPoints Q (orbit G P)) by
        exact Exists.elim this fun R hR => by
          rw [← Sylow.ext (H.mp hR)]
          exact R.2
      apply Q.2.nonempty_fixed_point_of_prime_not_dvd_card
      refine fun h => hp.out.not_dvd_one (Nat.modEq_zero_iff_dvd.mp ?_)
      calc
        1 = Nat.card (fixedPoints P (orbit G P)) := ?_
        _ ≡ Nat.card (orbit G P) [MOD p] := (P.2.card_modEq_card_fixedPoints (orbit G P)).symm
        _ ≡ 0 [MOD p] := Nat.modEq_zero_iff_dvd.mpr h
      rw [← Nat.card_unique (α := ({⟨P, mem_orbit_self P⟩} : Set (orbit G P))), eq_comm]
      congr
      rw [Set.eq_singleton_iff_unique_mem]
      exact ⟨H.mpr rfl, fun R h => Subtype.ext (Sylow.ext (H.mp h))⟩