-- Lean 4 proof step from Mathlib.Topology.MetricSpace.GromovHausdorff (hypothesis)
Goal:
t : Set GHSpace
C : ℝ
u : ℕ → ℝ
K : ℕ → ℕ
ulim : Tendsto u atTop (𝓝 0)
hdiam : ∀ p ∈ t, diam (α := p.Rep) univ ≤ C
hcov :
  ∀ p ∈ t,
    ∀ (n : ℕ),
      ∃ (s : Set p.Rep), #(↑s : Type) ≤ (↑(K n) : Cardinal.{0}) ∧ Subset (α := Set p.Rep) univ (⋃ x ∈ s, ball x (u n))
δ : ℝ
δpos : δ > 0
ε : ℝ := 1 / 5 * δ
εpos : 0 < ε
n : ℕ
hn : ∀ n_1 ≥ n, dist (u n_1) 0 < ε
u_le_ε : u n ≤ ε
s : (p : GHSpace) → Set p.Rep
N : (p : GHSpace) → ℕ
hN : ∀ (p : GHSpace), N p ≤ K n
E : (p : GHSpace) → (↑(s p) : Type) ≃ Fin (N p)
hs : ∀ p ∈ t, Subset (α := Set p.Rep) univ (⋃ x ∈ s p, ball x (u n))
M : ℕ := ⌊ε⁻¹ * max C 0⌋₊
F : GHSpace → (k : Fin (K n).succ) × (Fin (↑k : ℕ) → Fin (↑k : ℕ) → Fin M.succ) :=
  fun (p : GHSpace) ↦
    Sigma.mk (β := fun (k : Fin (K n).succ) ↦ Fin (↑k : ℕ) → Fin (↑k : ℕ) → Fin M.succ) ⟨N p, ⋯⟩
      fun (a b : Fin (↑(⟨N p, ⋯⟩ : Fin (K n).succ) : ℕ)) ↦
      ⟨min M
          ⌊ε⁻¹ *
              dist (α := (↑(s p) : Type)) (((E p).symm : (a : Fin (N p)) → (↑(s p) : Type)) a : (↑(s p) : Type))
                (((E p).symm : (a : Fin (N p)) → (↑(s p) : Type)) b : (↑(s p) : Type))⌋₊,
        ⋯⟩
p : GHSpace
pt : p ∈ t
q : GHSpace
qt : q ∈ t
hpq :
  Eq (α := (k : Fin (K n).succ) × (Fin (↑k : ℕ) → Fin (↑k : ℕ) → Fin M.succ))
    ((fun (p : (↑t : Type)) ↦ F (↑p : GHSpace)) (Subtype.mk (p := fun (x : GHSpace) ↦ x ∈ t) p pt))
    ((fun (p : (↑t : Type)) ↦ F (↑p : GHSpace)) (Subtype.mk (p := fun (x : GHSpace) ↦ x ∈ t) q qt))
Npq : N p = N q
Ψ : (↑(s p) : Type) → (↑(s q) : Type) :=
  fun (x : (↑(s p) : Type)) ↦
    ((E q).symm : (a : Fin (N q)) → (↑(s q) : Type)) (Fin.cast Npq ((E p : (a : (↑(s p) : Type)) → Fin (N p)) x))
Φ : (↑(s p) : Type) → q.Rep := fun (x : (↑(s p) : Type)) ↦ (↑(Ψ x) : q.Rep)
⊢ dist (↑(Subtype.mk (p := fun (x : GHSpace) ↦ x ∈ t) p pt) : GHSpace)
      (↑(Subtype.mk (p := fun (x : GHSpace) ↦ x ∈ t) q qt) : GHSpace) <
    δ
have main : ghDist p.Rep q.Rep ≤ ε + ε / 2 + ε
Tactic: have main : ghDist p.Rep q.Rep ≤ ε + ε / 2 + ε := by
    -- to prove the main inequality, argue that `s p` is `ε`-dense in `p`, and `s q` is `ε`-dense
    -- in `q`, and `s p` and `s q` are almost isometric. Then closeness follows
    -- from `ghDist_le_of_approx_subsets`
    refine ghDist_le_of_approx_subsets Φ ?_ ?_ ?_
    · show ∀ x : p.Rep, ∃ y ∈ s p, dist x y ≤ ε
      -- by construction, `s p` is `ε`-dense
      intro x
      have : x ∈ ⋃ y ∈ s p, ball y (u n) := (hs p pt) (mem_univ _)
      rcases mem_iUnion₂.1 this with ⟨y, ys, hy⟩
      exact ⟨y, ys, le_trans (le_of_lt hy) u_le_ε⟩
    · show ∀ x : q.Rep, ∃ z : s p, dist x (Φ z) ≤ ε
      -- by construction, `s q` is `ε`-dense, and it is the range of `Φ`
      intro x
      have : x ∈ ⋃ y ∈ s q, ball y (u n) := (hs q qt) (mem_univ _)
      rcases mem_iUnion₂.1 this with ⟨y, ys, hy⟩
      let i : ℕ := E q ⟨y, ys⟩
      let hi := ((E q) ⟨y, ys⟩).2
      have ihi_eq : (⟨i, hi⟩ : Fin (N q)) = (E q) ⟨y, ys⟩ := by rw [Fin.ext_iff, Fin.val_mk]
      have hiq : i < N q := hi
      have hip : i < N p := by rwa [Npq.symm] at hiq
      let z := (E p).symm ⟨i, hip⟩
      use z
      have C1 : (E p) z = ⟨i, hip⟩ := (E p).apply_symm_apply ⟨i, hip⟩
      have C2 : Fin.cast Npq ⟨i, hip⟩ = ⟨i, hi⟩ := rfl
      have C3 : (E q).symm ⟨i, hi⟩ = ⟨y, ys⟩ := by
        rw [ihi_eq]; exact (E q).symm_apply_apply ⟨y, ys⟩
      have : Φ z = y := by simp only [Ψ, Φ]; rw [C1, C2, C3]
      rw [this]
      exact le_trans (le_of_lt hy) u_le_ε
    · show ∀ x y : s p, |dist x y - dist (Φ x) (Φ y)| ≤ ε
      /- the distance between `x` and `y` is encoded in `F p`, and the distance between
            `Φ x` and `Φ y` (two points of `s q`) is encoded in `F q`, all this up to `ε`.
            As `F p = F q`, the distances are almost equal. -/
      intro x y
      have : dist (Φ x) (Φ y) = dist (Ψ x) (Ψ y) := rfl
      rw [this]
      -- introduce `i`, that codes both `x` and `Φ x` in `Fin (N p) = Fin (N q)`
      let i : ℕ := E p x
      have hip : i < N p := ((E p) x).2
      have hiq : i < N q := by rwa [Npq] at hip
      have i' : i = (E q) (Ψ x) := by simp only [i, Ψ, Equiv.apply_symm_apply, Fin.val_cast]
      -- introduce `j`, that codes both `y` and `Φ y` in `Fin (N p) = Fin (N q)`
      let j : ℕ := E p y
      have hjp : j < N p := ((E p) y).2
      have hjq : j < N q := by rwa [Npq] at hjp
      have j' : j = (E q) (Ψ y) := by simp only [j, Ψ, Equiv.apply_symm_apply, Fin.val_cast]
      -- Express `dist x y` in terms of `F p`
      have Ap : ((F p).2 ⟨i, hip⟩ ⟨j, hjp⟩).1 = ⌊ε⁻¹ * dist x y⌋₊ :=
        calc
          ((F p).2 ⟨i, hip⟩ ⟨j, hjp⟩).1 = ((F p).2 ((E p) x) ((E p) y)).1 := by
            congr
          _ = min M ⌊ε⁻¹ * dist x y⌋₊ := by simp only [F, (E p).symm_apply_apply]
          _ = ⌊ε⁻¹ * dist x y⌋₊ := by
            refine min_eq_right (Nat.floor_mono ?_)
            refine mul_le_mul_of_nonneg_left (le_trans ?_ (le_max_left _ _)) (inv_pos.2 εpos).le
            change dist (x : p.Rep) y ≤ C
            refine (dist_le_diam_of_mem isCompact_univ.isBounded (mem_univ _) (mem_univ _)).trans ?_
            exact hdiam p pt
      -- Express `dist (Φ x) (Φ y)` in terms of `F q`
      have Aq : ((F q).2 ⟨i, hiq⟩ ⟨j, hjq⟩).1 = ⌊ε⁻¹ * dist (Ψ x) (Ψ y)⌋₊ :=
        calc
          ((F q).2 ⟨i, hiq⟩ ⟨j, hjq⟩).1 = ((F q).2 ((E q) (Ψ x)) ((E q) (Ψ y))).1 := by
            congr!
          _ = min M ⌊ε⁻¹ * dist (Ψ x) (Ψ y)⌋₊ := by simp only [F, (E q).symm_apply_apply]
          _ = ⌊ε⁻¹ * dist (Ψ x) (Ψ y)⌋₊ := by
            refine min_eq_right (Nat.floor_mono ?_)
            refine mul_le_mul_of_nonneg_left (le_trans ?_ (le_max_left _ _)) (inv_pos.2 εpos).le
            change dist (Ψ x : q.Rep) (Ψ y) ≤ C
            refine (dist_le_diam_of_mem isCompact_univ.isBounded (mem_univ _) (mem_univ _)).trans ?_
            exact hdiam q qt
      -- use the equality between `F p` and `F q` to deduce that the distances have equal
      -- integer parts
      have : ((F p).2 ⟨i, hip⟩ ⟨j, hjp⟩).1 = ((F q).2 ⟨i, hiq⟩ ⟨j, hjq⟩).1 := by
        have hpq' : (F p).snd ≍ (F q).snd := (Sigma.mk.inj_iff.1 hpq).2
        rw [Fin.heq_fun₂_iff Npq Npq] at hpq'
        rw [← hpq']
      have : ⌊ε⁻¹ * dist x y⌋ = ⌊ε⁻¹ * dist (Ψ x) (Ψ y)⌋ := by
        rw [Ap, Aq] at this
        have D : 0 ≤ ⌊ε⁻¹ * dist x y⌋ :=
          floor_nonneg.2 (mul_nonneg (le_of_lt (inv_pos.2 εpos)) dist_nonneg)
        have D' : 0 ≤ ⌊ε⁻¹ * dist (Ψ x) (Ψ y)⌋ :=
          floor_nonneg.2 (mul_nonneg (le_of_lt (inv_pos.2 εpos)) dist_nonneg)
        rw [← Int.toNat_of_nonneg D, ← Int.toNat_of_nonneg D', Int.floor_toNat, Int.floor_toNat,
          this]
      -- deduce that the distances coincide up to `ε`, by a straightforward computation
      -- that should be automated
      have I :=
        calc
          |ε⁻¹| * |dist x y - dist (Ψ x) (Ψ y)| = |ε⁻¹ * (dist x y - dist (Ψ x) (Ψ y))| :=
            (abs_mul _ _).symm
          _ = |ε⁻¹ * dist x y - ε⁻¹ * dist (Ψ x) (Ψ y)| := by congr; ring
          _ ≤ 1 := le_of_lt (abs_sub_lt_one_of_floor_eq_floor this)
      calc
        |dist x y - dist (Ψ x) (Ψ y)| = ε * ε⁻¹ * |dist x y - dist (Ψ x) (Ψ y)| := by
          rw [mul_inv_cancel₀ (ne_of_gt εpos), one_mul]
        _ = ε * (|ε⁻¹| * |dist x y - dist (Ψ x) (Ψ y)|) := by
          rw [abs_of_nonneg (le_of_lt (inv_pos.2 εpos)), mul_assoc]
        _ ≤ ε * 1 := mul_le_mul_of_nonneg_left I (le_of_lt εpos)
        _ = ε := mul_one _