theorem limsup_eq_tendsto_sum_indicator_nat_atTop (s : ℕ → Set α) :
    limsup s atTop = { ω | Tendsto
      (fun n ↦ ∑ k in Finset.range n, (s (k + 1)).indicator (1 : α → ℕ) ω) atTop atTop } := by
  ext ω
  simp only [limsup_eq_iInf_iSup_of_nat, ge_iff_le, Set.iSup_eq_iUnion, Set.iInf_eq_iInter,
    Set.mem_iInter, Set.mem_iUnion, exists_prop]
  constructor
  · intro hω
    refine' tendsto_atTop_atTop_of_monotone' (fun n m hnm ↦ Finset.sum_mono_set_of_nonneg
      (fun i ↦ Set.indicator_nonneg (fun _ _ ↦ zero_le_one) _) (Finset.range_mono hnm)) _
    rintro ⟨i, h⟩
    simp only [mem_upperBounds, Set.mem_range, forall_exists_index, forall_apply_eq_imp_iff] at h
    induction' i with k hk
    · obtain ⟨j, hj₁, hj₂⟩ := hω 1
      refine' not_lt.2 (h <| j + 1)
        (lt_of_le_of_lt (Finset.sum_const_zero.symm : 0 = ∑ k in Finset.range (j + 1), 0).le _)
      refine' Finset.sum_lt_sum (fun m _ ↦ Set.indicator_nonneg (fun _ _ ↦ zero_le_one) _)
        ⟨j - 1, Finset.mem_range.2 (lt_of_le_of_lt (Nat.sub_le _ _) j.lt_succ_self), _⟩
      rw [Nat.sub_add_cancel hj₁, Set.indicator_of_mem hj₂]
      exact zero_lt_one
    · rw [imp_false] at hk
      push_neg at hk
      obtain ⟨i, hi⟩ := hk
      obtain ⟨j, hj₁, hj₂⟩ := hω (i + 1)
      replace hi : (∑ k in Finset.range i, (s (k + 1)).indicator 1 ω) = k + 1 :=
        le_antisymm (h i) hi
      refine' not_lt.2 (h <| j + 1) _
      rw [← Finset.sum_range_add_sum_Ico _ (i.le_succ.trans (hj₁.trans j.le_succ)), hi]
      refine' lt_add_of_pos_right _ _
      rw [(Finset.sum_const_zero.symm : 0 = ∑ k in Finset.Ico i (j + 1), 0)]
      refine' Finset.sum_lt_sum (fun m _ ↦ Set.indicator_nonneg (fun _ _ ↦ zero_le_one) _)
        ⟨j - 1, Finset.mem_Ico.2 ⟨(Nat.le_sub_iff_add_le (le_trans ((le_add_iff_nonneg_left _).2
          zero_le') hj₁)).2 hj₁, lt_of_le_of_lt (Nat.sub_le _ _) j.lt_succ_self⟩, _⟩
      rw [Nat.sub_add_cancel (le_trans ((le_add_iff_nonneg_left _).2 zero_le') hj₁),
        Set.indicator_of_mem hj₂]
      exact zero_lt_one
  · rintro hω i
    rw [Set.mem_setOf_eq, tendsto_atTop_atTop] at hω
    by_contra! hcon
    obtain ⟨j, h⟩ := hω (i + 1)
    have : (∑ k in Finset.range j, (s (k + 1)).indicator 1 ω) ≤ i := by
      have hle : ∀ j ≤ i, (∑ k in Finset.range j, (s (k + 1)).indicator 1 ω) ≤ i := by
        refine' fun j hij ↦
          (Finset.sum_le_card_nsmul _ _ _ _ : _ ≤ (Finset.range j).card • 1).trans _
        · exact fun m _ ↦ Set.indicator_apply_le' (fun _ ↦ le_rfl) fun _ ↦ zero_le_one
        · simpa only [Finset.card_range, smul_eq_mul, mul_one]
      by_cases hij : j < i
      · exact hle _ hij.le
      · rw [← Finset.sum_range_add_sum_Ico _ (not_lt.1 hij)]
        

Proof state:
case neg
ι : Type u_1
α : Type u_2
β : Type u_3
R : Type u_4
S : Type u_5
π : ι → Type u_6
s : ℕ → Set α
ω : α
hω : ∀ (b : ℕ), ∃ i, ∀ (a : ℕ), i ≤ a → b ≤ ∑ k in Finset.range a, Set.indicator (s (k + 1)) 1 ω
i : ℕ
hcon : ∀ (i_1 : ℕ), i ≤ i_1 → ω ∉ s i_1
j : ℕ
h : ∀ (a : ℕ), j ≤ a → i + 1 ≤ ∑ k in Finset.range a, Set.indicator (s (k + 1)) 1 ω
hle : ∀ j ≤ i, ∑ k in Finset.range j, Set.indicator (s (k + 1)) 1 ω ≤ i
hij : ¬j < i
⊢ ∑ k in Finset.range i, Set.indicator (s (k + 1)) 1 ω + ∑ k in Finset.Ico i j, Set.indicator (s (k + 1)) 1 ω ≤ i

Next tactic:
suffices (∑ k in Finset.Ico i j, (s (k + 1)).indicator 1 ω) = 0 by
          rw [this, add_zero]
          exact hle _ le_rfl