-- Lean 4 proof step from Mathlib.MeasureTheory.Function.UniformIntegrable (goal)
Goal:
α : Type u_1
β : Type u_2
ι : Type u_3
m : MeasurableSpace α
μ : Measure α
inst : NormedAddCommGroup β
p : ℝ≥0∞
hp : 1 ≤ p
hp' : p ≠ ∞
f : ι → α → β
hf : ∀ (i : ι), StronglyMeasurable (f i)
h :
  ∀ (ε : ℝ),
    0 < ε →
      ∃ (C : ℝ≥0),
        0 < C ∧ ∀ (i : ι), MeasureTheory.eLpNorm (indicator {x : α | C ≤ ‖f i x‖₊} (f i)) p μ ≤ ENNReal.ofReal ε
hpzero : p ≠ 0
hμ : ¬Eq (α := ℝ≥0∞) ((μ : (a : Set α) → ℝ≥0∞) univ : ℝ≥0∞) (0 : ℝ≥0∞)
ε : ℝ
hε : 0 < ε
C : ℝ≥0
hCpos : 0 < C
hC : ∀ (i : ι), MeasureTheory.eLpNorm (indicator {x : α | C ≤ ‖f i x‖₊} (f i)) p μ ≤ ENNReal.ofReal (ε / 2)
i : ι
s : Set α
hs : MeasurableSet s
hμs : (μ : (a : Set α) → ℝ≥0∞) s ≤ ENNReal.ofReal ((ε / (2 * (↑C : ℝ))) ^ p.toReal)
hμs' : ¬Eq (α := ℝ≥0∞) ((μ : (a : Set α) → ℝ≥0∞) s : ℝ≥0∞) (0 : ℝ≥0∞)
eLpNorm :
  MeasureTheory.eLpNorm (s.indicator (f i)) p μ ≤
    MeasureTheory.eLpNorm ((s ∩ {x : α | C ≤ ‖f i x‖₊}).indicator (f i)) p μ +
      MeasureTheory.eLpNorm ((s ∩ {x : α | ‖f i x‖₊ < C}).indicator (f i)) p μ
eLpNorm₁ :
  MeasureTheory.eLpNorm ((s ∩ {x : α | C ≤ ‖f i x‖₊}).indicator (f i)) p μ +
      MeasureTheory.eLpNorm ((s ∩ {x : α | ‖f i x‖₊ < C}).indicator (f i)) p μ ≤
    MeasureTheory.eLpNorm (indicator {x : α | C ≤ ‖f i x‖₊} (f i)) p μ +
      (↑C : ℝ≥0∞) * HPow.hPow (α := ℝ≥0∞) ((μ : (a : Set α) → ℝ≥0∞) s : ℝ≥0∞) (1 / p.toReal)
⊢ MeasureTheory.eLpNorm (s.indicator (f i)) p μ ≤ ENNReal.ofReal ε
have eLpNorm₂ : MeasureTheory.eLpNorm (indicator {x : α | C ≤ ‖f i x‖₊} (f i)) p μ +
    (↑C : ℝ≥0∞) * HPow.hPow (α := ℝ≥0∞) ((μ : (a : Set α) → ℝ≥0∞) s : ℝ≥0∞) (1 / p.toReal) ≤
  ENNReal.ofReal (ε / 2) + (↑C : ℝ≥0∞) * ENNReal.ofReal (ε / (2 * (↑C : ℝ)))
Tactic: calc
    eLpNorm (Set.indicator s (f i)) p μ ≤
        eLpNorm (Set.indicator (s ∩ { x | C ≤ ‖f i x‖₊ }) (f i)) p μ +
          eLpNorm (Set.indicator (s ∩ { x | ‖f i x‖₊ < C }) (f i)) p μ := by
      refine le_trans (Eq.le ?_) (eLpNorm_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]
      · rw [← Set.inter_union_distrib_left, (by ext; simp [le_or_gt] :
            { 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)
    _ ≤ eLpNorm (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
        (eLpNorm_mono fun x => norm_indicator_le_of_subset Set.inter_subset_right _ _) ?_
      rw [← Set.indicator_indicator]
      rw [eLpNorm_indicator_eq_eLpNorm_restrict hs]
      have : ∀ᵐ x ∂μ.restrict s, ‖{ x : α | ‖f i x‖₊ < C }.indicator (f i) x‖ ≤ C := by
        filter_upwards
        simp_rw [norm_indicator_eq_indicator_norm]
        exact Set.indicator_le' (fun x (hx : _ < _) => hx.le) fun _ _ => NNReal.coe_nonneg _
      refine le_trans (eLpNorm_le_of_ae_bound this) ?_
      rw [mul_comm, Measure.restrict_apply' hs, Set.univ_inter, ENNReal.ofReal_coe_nnreal, one_div]
    _ ≤ ENNReal.ofReal (ε / 2) + C * ENNReal.ofReal (ε / (2 * C)) := by
      grw [hC i]
      gcongr
      rwa [one_div, ENNReal.rpow_inv_le_iff (ENNReal.toReal_pos hpzero hp'),
        ENNReal.ofReal_rpow_of_pos (div_pos hε (mul_pos two_pos (NNReal.coe_pos.2 hCpos)))]
    _ ≤ ENNReal.ofReal (ε / 2) + ENNReal.ofReal (ε / 2) := by
      gcongr
      rw [← ENNReal.ofReal_coe_nnreal, ← ENNReal.ofReal_mul (NNReal.coe_nonneg _), ← div_div,
        mul_div_cancel₀ _ (NNReal.coe_pos.2 hCpos).ne.symm]
    _ ≤ ENNReal.ofReal ε := by
      rw [← ENNReal.ofReal_add (half_pos hε).le (half_pos hε).le, add_halves]