/-- Given an integrable function `g`, the conditional expectations of `g` with respect to
a sequence of sub-σ-algebras is uniformly integrable. -/
theorem Integrable.uniformIntegrable_condexp {ι : Type*} [IsFiniteMeasure μ] {g : α → ℝ}
    (hint : Integrable g μ) {ℱ : ι → MeasurableSpace α} (hℱ : ∀ i, ℱ i ≤ m0) :
    UniformIntegrable (fun i => μ[g|ℱ i]) 1 μ := by
  have hmeas : ∀ n, ∀ C, MeasurableSet {x | C ≤ ‖(μ[g|ℱ n]) x‖₊} := fun n C =>
    measurableSet_le measurable_const (stronglyMeasurable_condexp.mono (hℱ n)).measurable.nnnorm
  have hg : Memℒp g 1 μ := memℒp_one_iff_integrable.2 hint
  refine' uniformIntegrable_of le_rfl ENNReal.one_ne_top
    (fun n => (stronglyMeasurable_condexp.mono (hℱ n)).aestronglyMeasurable) fun ε hε => _
  by_cases hne : snorm g 1 μ = 0
  · rw [snorm_eq_zero_iff hg.1 one_ne_zero] at hne
    refine' ⟨0, fun n => (le_of_eq <|
      (snorm_eq_zero_iff ((stronglyMeasurable_condexp.mono (hℱ n)).aestronglyMeasurable.indicator
        (hmeas n 0)) one_ne_zero).2 _).trans (zero_le _)⟩
    filter_upwards [condexp_congr_ae (m := ℱ n) hne] with x hx
    simp only [zero_le', Set.setOf_true, Set.indicator_univ, Pi.zero_apply, hx, condexp_zero]
  obtain ⟨δ, hδ, h⟩ := hg.snorm_indicator_le μ le_rfl ENNReal.one_ne_top hε
  set C : ℝ≥0 := ⟨δ, hδ.le⟩⁻¹ * (snorm g 1 μ).toNNReal with hC
  have hCpos : 0 < C := mul_pos (inv_pos.2 hδ) (ENNReal.toNNReal_pos hne hg.snorm_lt_top.ne)
  have : ∀ n, μ {x : α | C ≤ ‖(μ[g|ℱ n]) x‖₊} ≤ ENNReal.ofReal δ := by
    intro n
    have := mul_meas_ge_le_pow_snorm' μ one_ne_zero ENNReal.one_ne_top
      ((stronglyMeasurable_condexp (m := ℱ n) (μ := μ) (f := g)).mono (hℱ n)).aestronglyMeasurable C
    rw [ENNReal.one_toReal, ENNReal.rpow_one, ENNReal.rpow_one, mul_comm, ←
      ENNReal.le_div_iff_mul_le (Or.inl (ENNReal.coe_ne_zero.2 hCpos.ne.symm))
        (Or.inl ENNReal.coe_lt_top.ne)] at this
    simp_rw [ENNReal.coe_le_coe] at this
    refine' this.trans _
    rw [ENNReal.div_le_iff_le_mul (Or.inl (ENNReal.coe_ne_zero.2 hCpos.ne.symm))
        (Or.inl ENNReal.coe_lt_top.ne),
      hC, Nonneg.inv_mk, ENNReal.coe_mul, ENNReal.coe_toNNReal hg.snorm_lt_top.ne, ← mul_assoc, ←
      ENNReal.ofReal_eq_coe_nnreal, ← ENNReal.ofReal_mul hδ.le, mul_inv_cancel hδ.ne.symm,
      ENNReal.ofReal_one, one_mul]
    exact snorm_one_condexp_le_snorm _
  refine' ⟨C, fun n => le_trans _ (h {x : α | C ≤ ‖(μ[g|ℱ n]) x‖₊} (hmeas n C) (this n))⟩
  have hmeasℱ : MeasurableSet[ℱ n] {x : α | C ≤ ‖(μ[g|ℱ n]) x‖₊} :=
    @measurableSet_le _ _ _ _ _ (ℱ n) _ _ _ _ _ measurable_const
      (@Measurable.nnnorm _ _ _ _ _ (ℱ n) _ stronglyMeasurable_condexp.measurable)
  

Proof state:
case neg.intro.intro
α : Type u_1
m m0 : MeasurableSpace α
μ : Measure α
ι : Type u_2
inst✝ : IsFiniteMeasure μ
g : α → ℝ
hint : Integrable g
ℱ : ι → MeasurableSpace α
hℱ : ∀ (i : ι), ℱ i ≤ m0
hmeas : ∀ (n : ι) (C : ℝ≥0), MeasurableSet {x | C ≤ ‖(μ[g|ℱ n]) x‖₊}
hg : Memℒp g 1
ε : ℝ
hε : 0 < ε
hne : ¬snorm g 1 μ = 0
δ : ℝ
hδ : 0 < δ
h : ∀ (s : Set α), MeasurableSet s → ↑↑μ s ≤ ENNReal.ofReal δ → snorm (Set.indicator s g) 1 μ ≤ ENNReal.ofReal ε
C : ℝ≥0 := { val := δ, property := (_ : 0 ≤ δ) }⁻¹ * ENNReal.toNNReal (snorm g 1 μ)
hC : C = { val := δ, property := (_ : 0 ≤ δ) }⁻¹ * ENNReal.toNNReal (snorm g 1 μ)
hCpos : 0 < C
this : ∀ (n : ι), ↑↑μ {x | C ≤ ‖(μ[g|ℱ n]) x‖₊} ≤ ENNReal.ofReal δ
n : ι
hmeasℱ : MeasurableSet {x | C ≤ ‖(μ[g|ℱ n]) x‖₊}
⊢ snorm (Set.indicator {x | C ≤ ‖(μ[g|ℱ n]) x‖₊} (μ[g|ℱ n])) 1 μ ≤ snorm (Set.indicator {x | C ≤ ‖(μ[g|ℱ n]) x‖₊} g) 1 μ

Next tactic:
rw [← snorm_congr_ae (condexp_indicator hint hmeasℱ)]