/-- If a function `f : α → β` is such that the level sets `{f < p}` and `{q < f}` have measurable
supersets which are disjoint up to measure zero when `p < q`, then `f` is almost-everywhere
measurable. It is even enough to have this for `p` and `q` in a countable dense set. -/
theorem MeasureTheory.aemeasurable_of_exist_almost_disjoint_supersets {α : Type*}
    {m : MeasurableSpace α} (μ : Measure α) {β : Type*} [CompleteLinearOrder β] [DenselyOrdered β]
    [TopologicalSpace β] [OrderTopology β] [SecondCountableTopology β] [MeasurableSpace β]
    [BorelSpace β] (s : Set β) (s_count : s.Countable) (s_dense : Dense s) (f : α → β)
    (h : ∀ p ∈ s, ∀ q ∈ s, p < q → ∃ u v, MeasurableSet u ∧ MeasurableSet v ∧
      { x | f x < p } ⊆ u ∧ { x | q < f x } ⊆ v ∧ μ (u ∩ v) = 0) :
    AEMeasurable f μ := by
  haveI : Encodable s := s_count.toEncodable
  have h' : ∀ p q, ∃ u v, MeasurableSet u ∧ MeasurableSet v ∧
      { x | f x < p } ⊆ u ∧ { x | q < f x } ⊆ v ∧ (p ∈ s → q ∈ s → p < q → μ (u ∩ v) = 0) := by
    intro p q
    by_cases H : p ∈ s ∧ q ∈ s ∧ p < q
    · rcases h p H.1 q H.2.1 H.2.2 with ⟨u, v, hu, hv, h'u, h'v, hμ⟩
      exact ⟨u, v, hu, hv, h'u, h'v, fun _ _ _ => hμ⟩
    · refine'
        ⟨univ, univ, MeasurableSet.univ, MeasurableSet.univ, subset_univ _, subset_univ _,
          fun ps qs pq => _⟩
      simp only [not_and] at H
      exact (H ps qs pq).elim
  choose! u v huv using h'
  let u' : β → Set α := fun p => ⋂ q ∈ s ∩ Ioi p, u p q
  have u'_meas : ∀ i, MeasurableSet (u' i) := by
    intro i
    exact MeasurableSet.biInter (s_count.mono (inter_subset_left _ _)) fun b _ => (huv i b).1
  let f' : α → β := fun x => ⨅ i : s, piecewise (u' i) (fun _ => (i : β)) (fun _ => (⊤ : β)) x
  have f'_meas : Measurable f' := by
    apply measurable_iInf
    exact fun i => Measurable.piecewise (u'_meas i) measurable_const measurable_const
  let t := ⋃ (p : s) (q : ↥(s ∩ Ioi p)), u' p ∩ v p q
  have μt : μ t ≤ 0 :=
    calc
      μ t ≤ ∑' (p : s) (q : ↥(s ∩ Ioi p)), μ (u' p ∩ v p q) := by
        refine (measure_iUnion_le _).trans ?_
        refine ENNReal.tsum_le_tsum fun p => ?_
        refine @measure_iUnion_le _ _ _ _ ?_ _
        exact (s_count.mono (inter_subset_left _ _)).to_subtype
      _ ≤ ∑' (p : s) (q : ↥(s ∩ Ioi p)), μ (u p q ∩ v p q) := by
        refine ENNReal.tsum_le_tsum fun p => ?_
        refine ENNReal.tsum_le_tsum fun q => measure_mono ?_
        exact inter_subset_inter_left _ (biInter_subset_of_mem q.2)
      _ = ∑' (p : s) (_ : ↥(s ∩ Ioi p)), (0 : ℝ≥0∞) := by
        congr
        ext1 p
        congr
        ext1 q
        exact (huv p q).2.2.2.2 p.2 q.2.1 q.2.2
      _ = 0 := by simp only [tsum_zero]
  have ff' : ∀ᵐ x ∂μ, f x = f' x := by
    have : ∀ᵐ x ∂μ, x ∉ t := by
      have : μ t = 0 := le_antisymm μt bot_le
      change μ _ = 0
      convert this
      ext y
      simp only [not_exists, exists_prop, mem_setOf_eq, mem_compl_iff, not_not_mem]
    filter_upwards [this] with x hx
    apply (iInf_eq_of_forall_ge_of_forall_gt_exists_lt _ _).symm
    · intro i
      by_cases H : x ∈ u' i
      swap
      · simp only [H, le_top, not_false_iff, piecewise_eq_of_not_mem]
      simp only [H, piecewise_eq_of_mem]
      contrapose! hx
      obtain ⟨r, ⟨xr, rq⟩, rs⟩ : ∃ r, r ∈ Ioo (i : β) (f x) ∩ s :=
        dense_iff_inter_open.1 s_dense (Ioo i (f x)) isOpen_Ioo (nonempty_Ioo.2 hx)
      have A : x ∈ v i r := (huv i r).2.2.2.1 rq
      refine mem_iUnion.2 ⟨i, ?_⟩
      refine mem_iUnion.2 ⟨⟨r, ⟨rs, xr⟩⟩, ?_⟩
      exact ⟨H, A⟩
    · intro q hq
      obtain ⟨r, ⟨xr, rq⟩, rs⟩ : ∃ r, r ∈ Ioo (f x) q ∩ s :=
        dense_iff_inter_open.1 s_dense (Ioo (f x) q) isOpen_Ioo (nonempty_Ioo.2 hq)
      refine' ⟨⟨r, rs⟩, _⟩
      

Proof state:
case intro.intro.intro
α : Type u_1
m : MeasurableSpace α
μ : Measure α
β : Type u_2
inst✝⁶ : CompleteLinearOrder β
inst✝⁵ : DenselyOrdered β
inst✝⁴ : TopologicalSpace β
inst✝³ : OrderTopology β
inst✝² : SecondCountableTopology β
inst✝¹ : MeasurableSpace β
inst✝ : BorelSpace β
s : Set β
s_count : Set.Countable s
s_dense : Dense s
f : α → β
h :
  ∀ p ∈ s,
    ∀ q ∈ s, p < q → ∃ u v, MeasurableSet u ∧ MeasurableSet v ∧ {x | f x < p} ⊆ u ∧ {x | q < f x} ⊆ v ∧ ↑↑μ (u ∩ v) = 0
this✝ : Encodable ↑s
u v : β → β → Set α
huv :
  ∀ (p q : β),
    MeasurableSet (u p q) ∧
      MeasurableSet (v p q) ∧
        {x | f x < p} ⊆ u p q ∧ {x | q < f x} ⊆ v p q ∧ (p ∈ s → q ∈ s → p < q → ↑↑μ (u p q ∩ v p q) = 0)
u' : β → Set α := fun p => ⋂ q ∈ s ∩ Ioi p, u p q
u'_meas : ∀ (i : β), MeasurableSet (u' i)
f' : α → β := fun x => ⨅ i, piecewise (u' ↑i) (fun x => ↑i) (fun x => ⊤) x
f'_meas : Measurable f'
t : Set α := ⋃ p, ⋃ q, u' ↑p ∩ v ↑p ↑q
μt : ↑↑μ t ≤ 0
this : ∀ᵐ (x : α) ∂μ, x ∉ t
x : α
hx : x ∉ t
q : β
hq : f x < q
r : β
rs : r ∈ s
xr : f x < r
rq : r < q
⊢ piecewise (u' ↑{ val := r, property := rs }) (fun x => ↑{ val := r, property := rs }) (fun x => ⊤) x < q

Next tactic:
have A : x ∈ u' r := mem_biInter fun i _ => (huv r i).2.2.1 xr