/-- In a space with the Besicovitch property, any set `s` can be covered with balls whose measures
add up to at most `μ s + ε`, for any positive `ε`. This works even if one restricts the set of
allowed radii around a point `x` to a set `f x` which accumulates at `0`. -/
theorem exists_closedBall_covering_tsum_measure_le (μ : Measure α) [SigmaFinite μ]
    [Measure.OuterRegular μ] {ε : ℝ≥0∞} (hε : ε ≠ 0) (f : α → Set ℝ) (s : Set α)
    (hf : ∀ x ∈ s, ∀ δ > 0, (f x ∩ Ioo 0 δ).Nonempty) :
    ∃ (t : Set α) (r : α → ℝ), t.Countable ∧ t ⊆ s ∧ (∀ x ∈ t, r x ∈ f x) ∧
      (s ⊆ ⋃ x ∈ t, closedBall x (r x)) ∧ (∑' x : t, μ (closedBall x (r x))) ≤ μ s + ε := by
  /- For the proof, first cover almost all `s` with disjoint balls thanks to the usual Besicovitch
    theorem. Taking the balls included in a well-chosen open neighborhood `u` of `s`, one may
    ensure that their measures add at most to `μ s + ε / 2`. Let `s'` be the remaining set, of
    measure `0`. Applying the other version of Besicovitch, one may cover it with at most `N`
    disjoint subfamilies. Making sure that they are all included in a neighborhood `v` of `s'` of
    measure at most `ε / (2 N)`, the sum of their measures is at most `ε / 2`,
    completing the proof. -/
  obtain ⟨u, su, u_open, μu⟩ : ∃ U, U ⊇ s ∧ IsOpen U ∧ μ U ≤ μ s + ε / 2 :=
    Set.exists_isOpen_le_add _ _
      (by
        simpa only [or_false, Ne.def, ENNReal.div_eq_zero_iff, ENNReal.two_ne_top] using hε)
  have : ∀ x ∈ s, ∃ R > 0, ball x R ⊆ u := fun x hx =>
    Metric.mem_nhds_iff.1 (u_open.mem_nhds (su hx))
  choose! R hR using this
  obtain ⟨t0, r0, t0_count, t0s, hr0, μt0, t0_disj⟩ :
    ∃ (t0 : Set α) (r0 : α → ℝ), t0.Countable ∧ t0 ⊆ s ∧
      (∀ x ∈ t0, r0 x ∈ f x ∩ Ioo 0 (R x)) ∧ μ (s \ ⋃ x ∈ t0, closedBall x (r0 x)) = 0 ∧
        t0.PairwiseDisjoint fun x => closedBall x (r0 x) :=
    exists_disjoint_closedBall_covering_ae μ f s hf R fun x hx => (hR x hx).1
  -- we have constructed an almost everywhere covering of `s` by disjoint balls. Let `s'` be the
  -- remaining set.
  let s' := s \ ⋃ x ∈ t0, closedBall x (r0 x)
  have s's : s' ⊆ s := diff_subset _ _
  obtain ⟨N, τ, hτ, H⟩ : ∃ N τ, 1 < τ ∧ IsEmpty (Besicovitch.SatelliteConfig α N τ) :=
    HasBesicovitchCovering.no_satelliteConfig
  obtain ⟨v, s'v, v_open, μv⟩ : ∃ v, v ⊇ s' ∧ IsOpen v ∧ μ v ≤ μ s' + ε / 2 / N :=
    Set.exists_isOpen_le_add _ _
      (by
        simp only [ne_eq, ENNReal.div_eq_zero_iff, hε, ENNReal.two_ne_top, or_self,
          ENNReal.nat_ne_top, not_false_eq_true])
  have : ∀ x ∈ s', ∃ r1 ∈ f x ∩ Ioo (0 : ℝ) 1, closedBall x r1 ⊆ v := by
    intro x hx
    rcases Metric.mem_nhds_iff.1 (v_open.mem_nhds (s'v hx)) with ⟨r, rpos, hr⟩
    rcases hf x (s's hx) (min r 1) (lt_min rpos zero_lt_one) with ⟨R', hR'⟩
    exact
      ⟨R', ⟨hR'.1, hR'.2.1, hR'.2.2.trans_le (min_le_right _ _)⟩,
        Subset.trans (closedBall_subset_ball (hR'.2.2.trans_le (min_le_left _ _))) hr⟩
  choose! r1 hr1 using this
  let q : BallPackage s' α :=
    { c := fun x => x
      r := fun x => r1 x
      rpos := fun x => (hr1 x.1 x.2).1.2.1
      r_bound := 1
      r_le := fun x => (hr1 x.1 x.2).1.2.2.le }
  -- by Besicovitch, we cover `s'` with at most `N` families of disjoint balls, all included in
  -- a suitable neighborhood `v` of `s'`.
  obtain ⟨S, S_disj, hS⟩ :
    ∃ S : Fin N → Set s',
      (∀ i : Fin N, (S i).PairwiseDisjoint fun j => closedBall (q.c j) (q.r j)) ∧
        range q.c ⊆ ⋃ i : Fin N, ⋃ j ∈ S i, ball (q.c j) (q.r j) :=
    exist_disjoint_covering_families hτ H q
  have S_count : ∀ i, (S i).Countable := by
    intro i
    apply (S_disj i).countable_of_nonempty_interior fun j _ => ?_
    have : (ball (j : α) (r1 j)).Nonempty := nonempty_ball.2 (q.rpos _)
    exact this.mono ball_subset_interior_closedBall
  let r x := if x ∈ s' then r1 x else r0 x
  have r_t0 : ∀ x ∈ t0, r x = r0 x := by
    intro x hx
    have : ¬x ∈ s' := by
      simp only [not_exists, exists_prop, mem_iUnion, mem_closedBall, not_and, not_lt, not_le,
        mem_diff, not_forall]
      intro _
      refine' ⟨x, hx, _⟩
      rw [dist_self]
      exact (hr0 x hx).2.1.le
    simp only [if_neg this]
  -- the desired covering set is given by the union of the families constructed in the first and
  -- second steps.
  refine' ⟨t0 ∪ ⋃ i : Fin N, ((↑) : s' → α) '' S i, r, _, _, _, _, _⟩
  -- it remains to check that they have the desired properties
  · exact t0_count.union (countable_iUnion fun i => (S_count i).image _)
  · simp only [t0s, true_and_iff, union_subset_iff, image_subset_iff, iUnion_subset_iff]
    intro i x _
    exact s's x.2
  · intro x hx
    cases hx with
    | inl hx =>
      rw [r_t0 x hx]
      exact (hr0 _ hx).1
    | inr hx =>
      have h'x : x ∈ s' := by
        simp only [mem_iUnion, mem_image] at hx
        rcases hx with ⟨i, y, _, rfl⟩
        exact y.2
      simp only [if_pos h'x, (hr1 x h'x).1.1]
  · intro x hx
    by_cases h'x : x ∈ s'
    · obtain ⟨i, y, ySi, xy⟩ : ∃ (i : Fin N) (y : ↥s'), y ∈ S i ∧ x ∈ ball (y : α) (r1 y) := by
        have A : x ∈ range q.c := by
          simpa only [not_exists, exists_prop, mem_iUnion, mem_closedBall, not_and, not_le,
            mem_setOf_eq, Subtype.range_coe_subtype, mem_diff] using h'x
        simpa only [mem_iUnion, mem_image, bex_def] using hS A
      refine' mem_iUnion₂.2 ⟨y, Or.inr _, _⟩
      · 

Proof state:
case pos.intro.intro.intro.refine'_1
α : Type u_1
inst✝⁶ : MetricSpace α
β : Type u
inst✝⁵ : SecondCountableTopology α
inst✝⁴ : MeasurableSpace α
inst✝³ : OpensMeasurableSpace α
inst✝² : HasBesicovitchCovering α
μ : Measure α
inst✝¹ : SigmaFinite μ
inst✝ : Measure.OuterRegular μ
ε : ℝ≥0∞
hε : ε ≠ 0
f : α → Set ℝ
s : Set α
hf : ∀ x ∈ s, ∀ δ > 0, Set.Nonempty (f x ∩ Ioo 0 δ)
u : Set α
su : u ⊇ s
u_open : IsOpen u
μu : ↑↑μ u ≤ ↑↑μ s + ε / 2
R : α → ℝ
hR : ∀ x ∈ s, R x > 0 ∧ ball x (R x) ⊆ u
t0 : Set α
r0 : α → ℝ
t0_count : Set.Countable t0
t0s : t0 ⊆ s
hr0 : ∀ x ∈ t0, r0 x ∈ f x ∩ Ioo 0 (R x)
μt0 : ↑↑μ (s \ ⋃ x ∈ t0, closedBall x (r0 x)) = 0
t0_disj : PairwiseDisjoint t0 fun x => closedBall x (r0 x)
s' : Set α := s \ ⋃ x ∈ t0, closedBall x (r0 x)
s's : s' ⊆ s
N : ℕ
τ : ℝ
hτ : 1 < τ
H : IsEmpty (SatelliteConfig α N τ)
v : Set α
s'v : v ⊇ s'
v_open : IsOpen v
μv : ↑↑μ v ≤ ↑↑μ s' + ε / 2 / ↑N
r1 : α → ℝ
hr1 : ∀ x ∈ s', r1 x ∈ f x ∩ Ioo 0 1 ∧ closedBall x (r1 x) ⊆ v
q : BallPackage (↑s') α :=
  { c := fun x => ↑x, r := fun x => r1 ↑x, rpos := (_ : ∀ (x : ↑s'), 0 < r1 ↑x), r_bound := 1,
    r_le := (_ : ∀ (x : ↑s'), r1 ↑x ≤ 1) }
S : Fin N → Set ↑s'
S_disj : ∀ (i : Fin N), PairwiseDisjoint (S i) fun j => closedBall (BallPackage.c q j) (BallPackage.r q j)
hS : range q.c ⊆ ⋃ i, ⋃ j ∈ S i, ball (BallPackage.c q j) (BallPackage.r q j)
S_count : ∀ (i : Fin N), Set.Countable (S i)
r : α → ℝ := fun x => if x ∈ s' then r1 x else r0 x
r_t0 : ∀ x ∈ t0, r x = r0 x
x : α
hx : x ∈ s
h'x : x ∈ s'
i : Fin N
y : ↑s'
ySi : y ∈ S i
xy : x ∈ ball (↑y) (r1 ↑y)
⊢ ↑y ∈ ⋃ i, Subtype.val '' S i

Next tactic:
simp only [mem_iUnion, mem_image]