/-- **Vitali covering theorem**: given a set `t` of subsets of a type, one may extract a disjoint
subfamily `u` such that the `τ`-enlargment of this family covers all elements of `t`, where `τ > 1`
is any fixed number.

When `t` is a family of balls, the `τ`-enlargment of `ball x r` is `ball x ((1+2τ) r)`. In general,
it is expressed in terms of a function `δ` (think "radius" or "diameter"), positive and bounded on
all elements of `t`. The condition is that every element `a` of `t` should intersect an
element `b` of `u` of size larger than that of `a` up to `τ`, i.e., `δ b ≥ δ a / τ`.

We state the lemma slightly more generally, with an indexed family of sets `B a` for `a ∈ t`, for
wider applicability.
-/
theorem exists_disjoint_subfamily_covering_enlargment (B : ι → Set α) (t : Set ι) (δ : ι → ℝ)
    (τ : ℝ) (hτ : 1 < τ) (δnonneg : ∀ a ∈ t, 0 ≤ δ a) (R : ℝ) (δle : ∀ a ∈ t, δ a ≤ R)
    (hne : ∀ a ∈ t, (B a).Nonempty) :
    ∃ (u : _) (_ : u ⊆ t),
      u.PairwiseDisjoint B ∧ ∀ a ∈ t, ∃ b ∈ u, (B a ∩ B b).Nonempty ∧ δ a ≤ τ * δ b := by
  /- The proof could be formulated as a transfinite induction. First pick an element of `t` with `δ`
  as large as possible (up to a factor of `τ`). Then among the remaining elements not intersecting
  the already chosen one, pick another element with large `δ`. Go on forever (transfinitely) until
  there is nothing left.

  Instead, we give a direct Zorn-based argument. Consider a maximal family `u` of disjoint sets
  with the following property: if an element `a` of `t` intersects some element `b` of `u`, then it
  intersects some `b' ∈ u` with `δ b' ≥ δ a / τ`. Such a maximal family exists by Zorn. If this
  family did not intersect some element `a ∈ t`, then take an element `a' ∈ t` which does not
  intersect any element of `u`, with `δ a'` almost as large as possible. One checks easily
  that `u ∪ {a'}` still has this property, contradicting the maximality. Therefore, `u`
  intersects all elements of `t`, and by definition it satisfies all the desired properties.
  -/
  let T : Set (Set ι) := { u | u ⊆ t ∧ u.PairwiseDisjoint B ∧
    ∀ a ∈ t, ∀ b ∈ u, (B a ∩ B b).Nonempty → ∃ c ∈ u, (B a ∩ B c).Nonempty ∧ δ a ≤ τ * δ c }
  -- By Zorn, choose a maximal family in the good set `T` of disjoint families.
  obtain ⟨u, uT, hu⟩ : ∃ u ∈ T, ∀ v ∈ T, u ⊆ v → v = u := by
    refine' zorn_subset _ fun U UT hU => _
    refine' ⟨⋃₀ U, _, fun s hs => subset_sUnion_of_mem hs⟩
    simp only [Set.sUnion_subset_iff, and_imp, exists_prop, forall_exists_index, mem_sUnion,
      Set.mem_setOf_eq]
    refine'
      ⟨fun u hu => (UT hu).1, (pairwiseDisjoint_sUnion hU.directedOn).2 fun u hu => (UT hu).2.1,
        fun a hat b u uU hbu hab => _⟩
    obtain ⟨c, cu, ac, hc⟩ : ∃ c, c ∈ u ∧ (B a ∩ B c).Nonempty ∧ δ a ≤ τ * δ c :=
      (UT uU).2.2 a hat b hbu hab
    exact ⟨c, ⟨u, uU, cu⟩, ac, hc⟩
  -- The only nontrivial bit is to check that every `a ∈ t` intersects an element `b ∈ u` with
  -- comparatively large `δ b`. Assume this is not the case, then we will contradict the maximality.
  

Proof state:
case intro.intro
α : Type u_1
ι : Type u_2
B : ι → Set α
t : Set ι
δ : ι → ℝ
τ : ℝ
hτ : 1 < τ
δnonneg : ∀ a ∈ t, 0 ≤ δ a
R : ℝ
δle : ∀ a ∈ t, δ a ≤ R
hne : ∀ a ∈ t, Set.Nonempty (B a)
T : Set (Set ι) :=
  {u |
    u ⊆ t ∧
      PairwiseDisjoint u B ∧
        ∀ a ∈ t, ∀ b ∈ u, Set.Nonempty (B a ∩ B b) → ∃ c ∈ u, Set.Nonempty (B a ∩ B c) ∧ δ a ≤ τ * δ c}
u : Set ι
uT : u ∈ T
hu : ∀ v ∈ T, u ⊆ v → v = u
⊢ ∃ u, ∃ (_ : u ⊆ t), PairwiseDisjoint u B ∧ ∀ a ∈ t, ∃ b ∈ u, Set.Nonempty (B a ∩ B b) ∧ δ a ≤ τ * δ b

Next tactic:
refine' ⟨u, uT.1, uT.2.1, fun a hat => _⟩