-- Module: Mathlib.MeasureTheory.Covering.Besicovitch
Goal:
α : Type u_1
inst : MetricSpace α
inst_1 : SecondCountableTopology α
inst_2 : MeasurableSpace α
inst_3 : OpensMeasurableSpace α
inst_4 : HasBesicovitchCovering α
μ : Measure α
inst_5 : SFinite μ
inst_6 : μ.OuterRegular
ε : ℝ≥0∞
hε : ε ≠ 0
f : α → Set ℝ
s : Set α
hf : ∀ x ∈ s, ∀ δ > 0, (f x ∩ Ioo 0 δ).Nonempty
u : Set α
su : u ⊇ s
u_open : IsOpen[PseudoMetricSpace.toUniformSpace.toTopologicalSpace] u
μu : (μ : (a : Set α) → ℝ≥0∞) u ≤ HAdd.hAdd (α := ℝ≥0∞) ((μ : (a : Set α) → ℝ≥0∞) s : ℝ≥0∞) (ε / 2)
R : (x : α) → ℝ
hR : ∀ x ∈ s, R x > 0 ∧ ball x (R x) ⊆ u
t0 : Set α
r0 : α → ℝ
t0_count : t0.Countable
t0s : t0 ⊆ s
hr0 : ∀ x ∈ t0, r0 x ∈ f x ∩ Ioo 0 (R x)
μt0 : Eq (α := ℝ≥0∞) ((μ : (a : Set α) → ℝ≥0∞) (s \ ⋃ x ∈ t0, closedBall x (r0 x)) : ℝ≥0∞) (0 : ℝ≥0∞)
t0_disj : t0.PairwiseDisjoint (α := Set α) fun (x : α) ↦ 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.{u_1 + 1} (SatelliteConfig α N τ)
v : Set α
s'v : v ⊇ s'
v_open : IsOpen[PseudoMetricSpace.toUniformSpace.toTopologicalSpace] v
μv : (μ : (a : Set α) → ℝ≥0∞) v ≤ HAdd.hAdd (α := ℝ≥0∞) ((μ : (a : Set α) → ℝ≥0∞) s' : ℝ≥0∞) (ε / 2 / (↑N : ℝ≥0∞))
r1 : (x : α) → ℝ
hr1 : ∀ x ∈ s', r1 x ∈ f x ∩ Ioo 0 1 ∧ closedBall x (r1 x) ⊆ v
q : BallPackage (↑s' : Type u_1) α :=
  { c := fun (x : (↑s' : Type u_1)) ↦ (↑x : α), r := fun (x : (↑s' : Type u_1)) ↦ r1 (↑x : α), rpos := ⋯, r_bound := 1,
    r_le := ⋯ }
S : Fin N → Set (↑s' : Type u_1)
S_disj : ∀ (i : Fin N), (S i).PairwiseDisjoint (α := Set α) fun (j : (↑s' : Type u_1)) ↦ closedBall (q.c j) (q.r j)
hS : range q.c ⊆ ⋃ (i : Fin N), ⋃ j ∈ S i, ball (q.c j) (q.r j)
S_count : ∀ (i : Fin N), (S i).Countable
r : (x : α) → ℝ := fun (x : α) ↦ if x ∈ s' then r1 x else r0 x
r_t0 : ∀ x ∈ t0, r x = r0 x
A :
  ∑' (x : (↑t0 : Type u_1)), (μ : (a : Set α) → ℝ≥0∞) (closedBall (↑x : α) (r (↑x : α))) ≤
    HAdd.hAdd (α := ℝ≥0∞) ((μ : (a : Set α) → ℝ≥0∞) s : ℝ≥0∞) (ε / 2)
⊢ ∀ (i : Fin N),
    ∑' (x : (↑(Subtype.val (p := fun (x : α) ↦ x ∈ s') '' S i) : Type u_1)),
        (μ : (a : Set α) → ℝ≥0∞) (closedBall (↑x : α) (r (↑x : α))) ≤
      ε / 2 / (↑N : ℝ≥0∞)

Tactic:
have B : ∀ i : Fin N, (∑' x : ((↑) : s' → α) '' S i, μ (closedBall x (r x))) ≤ ε / 2 / N :=
      fun i =>
      calc
        (∑' x : ((↑) : s' → α) '' S i, μ (closedBall x (r x))) =
            ∑' x : S i, μ (closedBall x (r x)) := by
          have : InjOn ((↑) : s' → α) (S i) := Subtype.val_injective.injOn
          let F : S i ≃ ((↑) : s' → α) '' S i := this.bijOn_image.equiv _
          exact (F.tsum_eq fun x => μ (closedBall x (r x))).symm
        _ = ∑' x : S i, μ (closedBall x (r1 x)) := by
          grind
        _ = μ (⋃ x : S i, closedBall x (r1 x)) := by
          haveI : Encodable (S i) := (S_count i).toEncodable
          rw [measure_iUnion]
          · exact (pairwise_subtype_iff_pairwise_set _ _).2 (S_disj i)
          · exact fun i => measurableSet_closedBall
        _ ≤ μ v := by
          apply measure_mono
          simp only [SetCoe.forall, iUnion_subset_iff]
          intro x xs' _
          exact (hr1 x xs').2
        _ ≤ ε / 2 / N := by have : μ s' = 0 := μt0; rwa [this, zero_add] at μv