/-- Consider, for each `x` in a set `s`, a radius `r x ∈ (0, 1]`. Then one can find finitely
many disjoint balls of the form `closedBall x (r x)` covering a proportion `1/(N+1)` of `s`, if
there are no satellite configurations with `N+1` points.
-/
theorem exist_finset_disjoint_balls_large_measure (μ : Measure α) [IsFiniteMeasure μ] {N : ℕ}
    {τ : ℝ} (hτ : 1 < τ) (hN : IsEmpty (SatelliteConfig α N τ)) (s : Set α) (r : α → ℝ)
    (rpos : ∀ x ∈ s, 0 < r x) (rle : ∀ x ∈ s, r x ≤ 1) :
    ∃ t : Finset α,
      ↑t ⊆ s ∧
        μ (s \ ⋃ x ∈ t, closedBall x (r x)) ≤ N / (N + 1) * μ s ∧
          (t : Set α).PairwiseDisjoint fun x => closedBall x (r x) := by
  -- exclude the trivial case where `μ s = 0`.
  rcases le_or_lt (μ s) 0 with (hμs | hμs)
  · have : μ s = 0 := le_bot_iff.1 hμs
    refine' ⟨∅, by simp only [Finset.coe_empty, empty_subset], _, _⟩
    · simp only [this, Finset.not_mem_empty, diff_empty, iUnion_false, iUnion_empty,
        nonpos_iff_eq_zero, mul_zero]
    · simp only [Finset.coe_empty, pairwiseDisjoint_empty]
  cases isEmpty_or_nonempty α
  · simp only [eq_empty_of_isEmpty s, measure_empty] at hμs
    exact (lt_irrefl _ hμs).elim
  have Npos : N ≠ 0 := by
    rintro rfl
    inhabit α
    exact not_isEmpty_of_nonempty _ hN
  -- introduce a measurable superset `o` with the same measure, for measure computations
  obtain ⟨o, so, omeas, μo⟩ : ∃ o : Set α, s ⊆ o ∧ MeasurableSet o ∧ μ o = μ s :=
    exists_measurable_superset μ s
  /- We will apply the topological Besicovitch theorem, giving `N` disjoint subfamilies of balls
    covering `s`. Among these, one of them covers a proportion at least `1/N` of `s`. A large
    enough finite subfamily will then cover a proportion at least `1/(N+1)`. -/
  let a : BallPackage s α :=
    { c := fun x => x
      r := fun x => r x
      rpos := fun x => rpos x x.2
      r_bound := 1
      r_le := fun x => rle x x.2 }
  rcases exist_disjoint_covering_families hτ hN a with ⟨u, hu, hu'⟩
  have u_count : ∀ i, (u i).Countable := by
    intro i
    refine' (hu i).countable_of_nonempty_interior fun j _ => _
    have : (ball (j : α) (r j)).Nonempty := nonempty_ball.2 (a.rpos _)
    exact this.mono ball_subset_interior_closedBall
  let v : Fin N → Set α := fun i => ⋃ (x : s) (_ : x ∈ u i), closedBall x (r x)
  have A : s = ⋃ i : Fin N, s ∩ v i := by
    refine' Subset.antisymm _ (iUnion_subset fun i => inter_subset_left _ _)
    intro x hx
    obtain ⟨i, y, hxy, h'⟩ :
        ∃ (i : Fin N) (i_1 : ↥s), i_1 ∈ u i ∧ x ∈ ball (↑i_1) (r ↑i_1) := by
      have : x ∈ range a.c := by simpa only [Subtype.range_coe_subtype, setOf_mem_eq]
      simpa only [mem_iUnion, bex_def] using hu' this
    

Proof state:
case intro.intro.intro
α : Type u_1
inst✝⁴ : MetricSpace α
β : Type u
inst✝³ : SecondCountableTopology α
inst✝² : MeasurableSpace α
inst✝¹ : OpensMeasurableSpace α
μ : Measure α
inst✝ : IsFiniteMeasure μ
N : ℕ
τ : ℝ
hτ : 1 < τ
hN : IsEmpty (SatelliteConfig α N τ)
s : Set α
r : α → ℝ
rpos : ∀ x ∈ s, 0 < r x
rle : ∀ x ∈ s, r x ≤ 1
hμs : 0 < ↑↑μ s
h✝ : Nonempty α
Npos : N ≠ 0
o : Set α
so : s ⊆ o
omeas : MeasurableSet o
μo : ↑↑μ o = ↑↑μ s
a : BallPackage (↑s) α :=
  { c := fun x => ↑x, r := fun x => r ↑x, rpos := (_ : ∀ (x : ↑s), 0 < r ↑x), r_bound := 1,
    r_le := (_ : ∀ (x : ↑s), r ↑x ≤ 1) }
u : Fin N → Set ↑s
hu : ∀ (i : Fin N), PairwiseDisjoint (u i) fun j => closedBall (BallPackage.c a j) (BallPackage.r a j)
hu' : range a.c ⊆ ⋃ i, ⋃ j ∈ u i, ball (BallPackage.c a j) (BallPackage.r a j)
u_count : ∀ (i : Fin N), Set.Countable (u i)
v : Fin N → Set α := fun i => ⋃ x ∈ u i, closedBall (↑x) (r ↑x)
x : α
hx : x ∈ s
i : Fin N
y : ↑s
hxy : y ∈ u i
h' : x ∈ ball (↑y) (r ↑y)
⊢ x ∈ ⋃ i, s ∩ v i

Next tactic:
refine' mem_iUnion.2 ⟨i, ⟨hx, _⟩⟩