/-- Assume that around every point there are arbitrarily small scales at which the measure is
doubling. Then the set of closed sets `a` with nonempty interior contained in `closedBall x r` and
covering a fixed proportion `1/C` of the ball `closedBall x (3 * r)` forms a Vitali family.
This is essentially a restatement of the measurable Vitali theorem. -/
protected def vitaliFamily [MetricSpace α] [MeasurableSpace α] [OpensMeasurableSpace α]
    [SecondCountableTopology α] (μ : Measure α) [IsLocallyFiniteMeasure μ] (C : ℝ≥0)
    (h : ∀ x, ∃ᶠ r in 𝓝[>] 0, μ (closedBall x (3 * r)) ≤ C * μ (closedBall x r)) :
    VitaliFamily μ where
  setsAt x := { a | IsClosed a ∧ (interior a).Nonempty ∧
    ∃ r, a ⊆ closedBall x r ∧ μ (closedBall x (3 * r)) ≤ C * μ a }
  MeasurableSet' x a ha := ha.1.measurableSet
  nonempty_interior x a ha := ha.2.1
  Nontrivial x ε εpos := by
    obtain ⟨r, μr, rpos, rε⟩ :
        ∃ r, μ (closedBall x (3 * r)) ≤ C * μ (closedBall x r) ∧ r ∈ Ioc (0 : ℝ) ε :=
      ((h x).and_eventually (Ioc_mem_nhdsWithin_Ioi ⟨le_rfl, εpos⟩)).exists
    refine'
      ⟨closedBall x r, ⟨isClosed_ball, _, ⟨r, Subset.rfl, μr⟩⟩, closedBall_subset_closedBall rε⟩
    exact (nonempty_ball.2 rpos).mono ball_subset_interior_closedBall
  covering := by
    intro s f fsubset ffine
    let t : Set (ℝ × α × Set α) :=
      { p | p.2.2 ⊆ closedBall p.2.1 p.1 ∧ μ (closedBall p.2.1 (3 * p.1)) ≤ C * μ p.2.2 ∧
            (interior p.2.2).Nonempty ∧ IsClosed p.2.2 ∧ p.2.2 ∈ f p.2.1 ∧ p.2.1 ∈ s }
    have A : ∀ x ∈ s, ∀ ε : ℝ, ε > 0 → ∃ p, p ∈ t ∧ p.1 ≤ ε ∧ p.2.1 = x := by
      intro x xs ε εpos
      rcases ffine x xs ε εpos with ⟨a, ha, h'a⟩
      rcases fsubset x xs ha with ⟨a_closed, a_int, ⟨r, ar, μr⟩⟩
      refine' ⟨⟨min r ε, x, a⟩, ⟨_, _, a_int, a_closed, ha, xs⟩, min_le_right _ _, rfl⟩
      · rcases min_cases r ε with (h' | h') <;> rwa [h'.1]
      · apply le_trans (measure_mono (closedBall_subset_closedBall _)) μr
        exact mul_le_mul_of_nonneg_left (min_le_left _ _) zero_le_three
    rcases exists_disjoint_covering_ae μ s t C (fun p => p.1) (fun p => p.2.1) (fun p => p.2.2)
        (fun p hp => hp.1) (fun p hp => hp.2.1) (fun p hp => hp.2.2.1) (fun p hp => hp.2.2.2.1) A
      with ⟨t', t't, _, t'_disj, μt'⟩
    refine' ⟨(fun p : ℝ × α × Set α => p.2) '' t', _, _, _, _⟩
    · rintro - ⟨q, hq, rfl⟩
      exact (t't hq).2.2.2.2.2
    · rintro p ⟨q, hq, rfl⟩ p' ⟨q', hq', rfl⟩ hqq'
      exact t'_disj hq hq' (ne_of_apply_ne _ hqq')
    · rintro - ⟨q, hq, rfl⟩
      exact (t't hq).2.2.2.2.1
    · convert μt' using 3
      

Proof state:
case h.e'_2.h.e'_3.h.e'_4
α : Type u_1
ι : Type u_2
inst✝⁴ : MetricSpace α
inst✝³ : MeasurableSpace α
inst✝² : OpensMeasurableSpace α
inst✝¹ : SecondCountableTopology α
μ : Measure α
inst✝ : IsLocallyFiniteMeasure μ
C : ℝ≥0
h : ∀ (x : α), ∃ᶠ (r : ℝ) in 𝓝[>] 0, ↑↑μ (closedBall x (3 * r)) ≤ ↑C * ↑↑μ (closedBall x r)
s : Set α
f : α → Set (Set α)
fsubset :
  ∀ x ∈ s,
    f x ⊆
      (fun x =>
          {a |
            IsClosed a ∧ Set.Nonempty (interior a) ∧ ∃ r, a ⊆ closedBall x r ∧ ↑↑μ (closedBall x (3 * r)) ≤ ↑C * ↑↑μ a})
        x
ffine : ∀ x ∈ s, ∀ ε > 0, ∃ a ∈ f x, a ⊆ closedBall x ε
t : Set (ℝ × α × Set α) :=
  {p |
    p.2.2 ⊆ closedBall p.2.1 p.1 ∧
      ↑↑μ (closedBall p.2.1 (3 * p.1)) ≤ ↑C * ↑↑μ p.2.2 ∧
        Set.Nonempty (interior p.2.2) ∧ IsClosed p.2.2 ∧ p.2.2 ∈ f p.2.1 ∧ p.2.1 ∈ s}
A : ∀ x ∈ s, ∀ ε > 0, ∃ p ∈ t, p.1 ≤ ε ∧ p.2.1 = x
t' : Set (ℝ × α × Set α)
t't : t' ⊆ t
left✝ : Set.Countable t'
t'_disj : PairwiseDisjoint t' fun p => p.2.2
μt' : ↑↑μ (s \ ⋃ a ∈ t', a.2.2) = 0
⊢ ⋃ p ∈ (fun p => p.2) '' t', p.2 = ⋃ a ∈ t', a.2.2

Next tactic:
rw [biUnion_image]