/-- This lemma is superceded by `unifIntegrable_of` which do not require `C` to be positive. -/
theorem unifIntegrable_of' (hp : 1 ≤ p) (hp' : p ≠ ∞) {f : ι → α → β}
    (hf : ∀ i, StronglyMeasurable (f i))
    (h : ∀ ε : ℝ, 0 < ε → ∃ C : ℝ≥0, 0 < C ∧
      ∀ i, snorm ({ x | C ≤ ‖f i x‖₊ }.indicator (f i)) p μ ≤ ENNReal.ofReal ε) :
    UnifIntegrable f p μ := by
  have hpzero := (lt_of_lt_of_le zero_lt_one hp).ne.symm
  by_cases hμ : μ Set.univ = 0
  · rw [Measure.measure_univ_eq_zero] at hμ
    exact hμ.symm ▸ unifIntegrable_zero_meas
  intro ε hε
  obtain ⟨C, hCpos, hC⟩ := h (ε / 2) (half_pos hε)
  refine' ⟨(ε / (2 * C)) ^ ENNReal.toReal p,
    Real.rpow_pos_of_pos (div_pos hε (mul_pos two_pos (NNReal.coe_pos.2 hCpos))) _,
    fun i s hs hμs => _⟩
  by_cases hμs' : μ s = 0
  · rw [(snorm_eq_zero_iff ((hf i).indicator hs).aestronglyMeasurable hpzero).2
        (indicator_meas_zero hμs')]
    norm_num
  calc
    snorm (Set.indicator s (f i)) p μ ≤
        snorm (Set.indicator (s ∩ { x | C ≤ ‖f i x‖₊ }) (f i)) p μ +
          snorm (Set.indicator (s ∩ { x | ‖f i x‖₊ < C }) (f i)) p μ := by
      refine' le_trans (Eq.le _) (snorm_add_le
        (StronglyMeasurable.aestronglyMeasurable
          ((hf i).indicator (hs.inter (stronglyMeasurable_const.measurableSet_le (hf i).nnnorm))))
        (StronglyMeasurable.aestronglyMeasurable
          ((hf i).indicator (hs.inter ((hf i).nnnorm.measurableSet_lt stronglyMeasurable_const))))
        hp)
      congr
      change _ = fun x => (s ∩ { x : α | C ≤ ‖f i x‖₊ }).indicator (f i) x +
        (s ∩ { x : α | ‖f i x‖₊ < C }).indicator (f i) x
      rw [← Set.indicator_union_of_disjoint]
      · congr
        rw [← Set.inter_union_distrib_left, (by ext; simp [le_or_lt] :
            { x : α | C ≤ ‖f i x‖₊ } ∪ { x : α | ‖f i x‖₊ < C } = Set.univ),
          Set.inter_univ]
      · refine' (Disjoint.inf_right' _ _).inf_left' _
        rw [disjoint_iff_inf_le]
        rintro x ⟨hx₁, hx₂⟩
        rw [Set.mem_setOf_eq] at hx₁ hx₂
        exact False.elim (hx₂.ne (eq_of_le_of_not_lt hx₁ (not_lt.2 hx₂.le)).symm)
    _ ≤ snorm (Set.indicator { x | C ≤ ‖f i x‖₊ } (f i)) p μ +
        (C : ℝ≥0∞) * μ s ^ (1 / ENNReal.toReal p) := by
      refine' add_le_add
        (snorm_mono fun x => norm_indicator_le_of_subset (Set.inter_subset_right _ _) _ _) _
      rw [← Set.indicator_indicator]
      rw [snorm_indicator_eq_snorm_restrict hs]
      have : ∀ᵐ x ∂μ.restrict s, ‖{ x : α | ‖f i x‖₊ < C }.indicator (f i) x‖ ≤ C := by
        refine' ae_of_all _ _
        simp_rw [norm_indicator_eq_indicator_norm]
        exact Set.indicator_le' (fun x (hx : _ < _) => hx.le) fun _ _ => NNReal.coe_nonneg _
      refine' le_trans (snorm_le_of_ae_bound this) _
      

Proof state:
α : Type u_1
β : Type u_2
ι : Type u_3
m : MeasurableSpace α
μ : Measure α
inst✝ : NormedAddCommGroup β
p : ℝ≥0∞
f✝ : ℕ → α → β
g : α → β
hp : 1 ≤ p
hp' : p ≠ ⊤
f : ι → α → β
hf : ∀ (i : ι), StronglyMeasurable (f i)
h : ∀ (ε : ℝ), 0 < ε → ∃ C, 0 < C ∧ ∀ (i : ι), snorm (indicator {x | C ≤ ‖f i x‖₊} (f i)) p μ ≤ ENNReal.ofReal ε
hpzero : p ≠ 0
hμ : ¬↑↑μ univ = 0
ε : ℝ
hε : 0 < ε
C : ℝ≥0
hCpos : 0 < C
hC : ∀ (i : ι), snorm (indicator {x | C ≤ ‖f i x‖₊} (f i)) p μ ≤ ENNReal.ofReal (ε / 2)
i : ι
s : Set α
hs : MeasurableSet s
hμs : ↑↑μ s ≤ ENNReal.ofReal ((ε / (2 * ↑C)) ^ ENNReal.toReal p)
hμs' : ¬↑↑μ s = 0
this : ∀ᵐ (x : α) ∂Measure.restrict μ s, ‖indicator {x | ‖f i x‖₊ < C} (f i) x‖ ≤ ↑C
⊢ ↑↑(Measure.restrict μ s) univ ^ (ENNReal.toReal p)⁻¹ * ENNReal.ofReal ↑C ≤ ↑C * ↑↑μ s ^ (1 / ENNReal.toReal p)

Next tactic:
rw [mul_comm, Measure.restrict_apply' hs, Set.univ_inter, ENNReal.ofReal_coe_nnreal, one_div]