-- Lean 4 proof step from Mathlib.MeasureTheory.Covering.Vitali (goal)
Goal:
α : Type u_1
ι : Type u_2
inst : PseudoMetricSpace α
inst_1 : MeasurableSpace α
inst_2 : OpensMeasurableSpace α
inst_3 : SecondCountableTopology α
μ : Measure α
inst_4 : IsLocallyFiniteMeasure μ
s : Set α
t : Set ι
C : ℝ≥0
r : ι → ℝ
c : ι → α
B : ι → Set α
hB : ∀ a ∈ t, B a ⊆ closedBall (c a) (r a)
μB :
  ∀ a ∈ t,
    (μ : (a : Set α) → ℝ≥0∞) (closedBall (c a) (3 * r a)) ≤
      HMul.hMul (β := ℝ≥0∞) (↑C : ℝ≥0∞) ((μ : (a : Set α) → ℝ≥0∞) (B a) : ℝ≥0∞)
ht : ∀ a ∈ t, (interior (B a)).Nonempty
h't : ∀ a ∈ t, IsClosed[PseudoMetricSpace.toUniformSpace.toTopologicalSpace] (B a)
hf : ∀ x ∈ s, ∀ ε > 0, ∃ a ∈ t, r a ≤ ε ∧ c a = x
R : (x : α) → ℝ
hR0 : ∀ (x : α), 0 < R x
hR1 : ∀ (x : α), R x ≤ 1
hRμ : ∀ (x : α), LT.lt (α := ℝ≥0∞) ((μ : (a : Set α) → ℝ≥0∞) (closedBall x (20 * R x)) : ℝ≥0∞) (∞ : ℝ≥0∞)
t' : Set ι := {a : ι | a ∈ t ∧ r a ≤ R (c a)}
u : Set ι
ut' : u ⊆ t'
u_disj : u.PairwiseDisjoint B
hu : ∀ a ∈ t', ∃ b ∈ u, (B a ∩ B b).Nonempty ∧ r a ≤ 2 * r b
ut : u ⊆ t
u_count : u.Countable
x : α
h : x ∈ s \ ⋃ a ∈ u, B a
v : Set ι := {a : ι | a ∈ u ∧ (B a ∩ ball x (R x)).Nonempty}
vu : v ⊆ u
⊢ ∃ u_1 ∈ 𝓝[s \ ⋃ a ∈ u, B a] x, Eq (α := ℝ≥0∞) ((μ : (a : Set α) → ℝ≥0∞) u_1 : ℝ≥0∞) (0 : ℝ≥0∞)
have obtain : ∀ (K : ℝ) (μK : LT.lt (α := ℝ≥0∞) ((μ : (a : Set α) → ℝ≥0∞) (closedBall x K) : ℝ≥0∞) (∞ : ℝ≥0∞))
  (hK : ∀ a ∈ u, (B a ∩ ball x (R x)).Nonempty → B a ⊆ closedBall x K),
  ∃ u_1 ∈ 𝓝[s \ ⋃ a ∈ u, B a] x, Eq (α := ℝ≥0∞) ((μ : (a : Set α) → ℝ≥0∞) u_1 : ℝ≥0∞) (0 : ℝ≥0∞)
Tactic: obtain ⟨K, μK, hK⟩ : ∃ K, μ (closedBall x K) < ∞ ∧
      ∀ a ∈ u, (B a ∩ ball x (R x)).Nonempty → B a ⊆ closedBall x K := by
    have Idist_v : ∀ a ∈ v, dist (c a) x ≤ r a + R x := by
      intro a hav
      apply dist_le_add_of_nonempty_closedBall_inter_closedBall
      refine hav.2.mono ?_
      gcongr
      exacts [hB a (ut (vu hav)), ball_subset_closedBall]
    set R0 := sSup (r '' v) with R0_def
    have R0_bdd : BddAbove (r '' v) := by
      refine ⟨1, fun r' hr' => ?_⟩
      rcases (mem_image _ _ _).1 hr' with ⟨b, hb, rfl⟩
      exact le_trans (ut' (vu hb)).2 (hR1 (c b))
    rcases le_total R0 (R x) with (H | H)
    · refine ⟨20 * R x, hRμ x, fun a au hax => ?_⟩
      refine (hB a (ut au)).trans ?_
      apply closedBall_subset_closedBall'
      have : r a ≤ R0 := le_csSup R0_bdd (mem_image_of_mem _ ⟨au, hax⟩)
      linarith [Idist_v a ⟨au, hax⟩, hR0 x]
    · have R0pos : 0 < R0 := (hR0 x).trans_le H
      have vnonempty : v.Nonempty := by
        by_contra h
        rw [nonempty_iff_ne_empty, Classical.not_not] at h
        rw [h, image_empty, Real.sSup_empty] at R0_def
        exact lt_irrefl _ (R0pos.trans_le (le_of_eq R0_def))
      obtain ⟨a, hav, R0a⟩ : ∃ a ∈ v, R0 / 2 < r a := by
        obtain ⟨r', r'mem, hr'⟩ : ∃ r' ∈ r '' v, R0 / 2 < r' :=
          exists_lt_of_lt_csSup (vnonempty.image _) (half_lt_self R0pos)
        rcases (mem_image _ _ _).1 r'mem with ⟨a, hav, rfl⟩
        exact ⟨a, hav, hr'⟩
      refine ⟨8 * R0, ?_, ?_⟩
      · apply lt_of_le_of_lt (measure_mono _) (hRμ (c a))
        apply closedBall_subset_closedBall'
        rw [dist_comm]
        linarith [Idist_v a hav, (ut' (vu hav)).2]
      · intro b bu hbx
        refine (hB b (ut bu)).trans ?_
        apply closedBall_subset_closedBall'
        have : r b ≤ R0 := le_csSup R0_bdd (mem_image_of_mem _ ⟨bu, hbx⟩)
        linarith [Idist_v b ⟨bu, hbx⟩]