-- Lean 4 proof step from Mathlib.Topology.MetricSpace.GromovHausdorff (hypothesis)
Goal:
X : Type u
inst : MetricSpace X
inst_1 : CompactSpace X
inst_2 : Nonempty X
Y : Type v
inst_3 : MetricSpace Y
inst_4 : CompactSpace Y
inst_5 : Nonempty Y
inhabited_h_1 : Inhabited X
inhabited_h : Inhabited Y
⊢ hausdorffDist (α := OptimalGHCoupling X Y) (range (optimalGHInjl X Y)) (range (optimalGHInjr X Y)) = ghDist X Y
have A : ∀ (p q : NonemptyCompacts ↥(lp (fun (n : ℕ) ↦ ℝ) ∞)),
  ⟦p⟧ = toGHSpace X →
    ⟦q⟧ = toGHSpace Y →
      hausdorffDist (↑p : Set ↥(lp (fun (n : ℕ) ↦ ℝ) ∞)) (↑q : Set ↥(lp (fun (n : ℕ) ↦ ℝ) ∞)) <
          diam.{u} (α := X) univ + 1 + diam.{v} (α := Y) univ →
        hausdorffDist (α := OptimalGHCoupling X Y) (range (optimalGHInjl X Y)) (range (optimalGHInjr X Y)) ≤
          hausdorffDist (↑p : Set ↥(lp (fun (n : ℕ) ↦ ℝ) ∞)) (↑q : Set ↥(lp (fun (n : ℕ) ↦ ℝ) ∞))
Tactic: have A :
    ∀ p q : NonemptyCompacts ℓ_infty_ℝ,
      ⟦p⟧ = toGHSpace X →
        ⟦q⟧ = toGHSpace Y →
          hausdorffDist (p : Set ℓ_infty_ℝ) q < diam (univ : Set X) + 1 + diam (univ : Set Y) →
            hausdorffDist (range (optimalGHInjl X Y)) (range (optimalGHInjr X Y)) ≤
              hausdorffDist (p : Set ℓ_infty_ℝ) q := by
    intro p q hp hq bound
    rcases eq_toGHSpace_iff.1 hp with ⟨Φ, ⟨Φisom, Φrange⟩⟩
    rcases eq_toGHSpace_iff.1 hq with ⟨Ψ, ⟨Ψisom, Ψrange⟩⟩
    have I : diam (range Φ ∪ range Ψ) ≤ 2 * diam (univ : Set X) + 1 + 2 * diam (univ : Set Y) := by
      rcases exists_mem_of_nonempty X with ⟨xX, _⟩
      have : ∃ y ∈ range Ψ, dist (Φ xX) y < diam (univ : Set X) + 1 + diam (univ : Set Y) := by
        rw [Ψrange]
        have : Φ xX ∈ (p : Set _) := Φrange ▸ (mem_range_self _)
        exact
          exists_dist_lt_of_hausdorffDist_lt this bound
            (hausdorffEDist_ne_top_of_nonempty_of_bounded p.nonempty q.nonempty
              p.isCompact.isBounded q.isCompact.isBounded)
      rcases this with ⟨y, hy, dy⟩
      rcases mem_range.1 hy with ⟨z, hzy⟩
      rw [← hzy] at dy
      have DΦ : diam (range Φ) = diam (univ : Set X) := Φisom.diam_range
      have DΨ : diam (range Ψ) = diam (univ : Set Y) := Ψisom.diam_range
      calc
        diam (range Φ ∪ range Ψ) ≤ diam (range Φ) + dist (Φ xX) (Ψ z) + diam (range Ψ) :=
          diam_union (mem_range_self _) (mem_range_self _)
        _ ≤
            diam (univ : Set X) + (diam (univ : Set X) + 1 + diam (univ : Set Y)) +
              diam (univ : Set Y) := by
          rw [DΦ, DΨ]
          gcongr
        _ = 2 * diam (univ : Set X) + 1 + 2 * diam (univ : Set Y) := by ring
    let f : X ⊕ Y → ℓ_infty_ℝ := fun x =>
      match x with
      | inl y => Φ y
      | inr z => Ψ z
    let F : (X ⊕ Y) × (X ⊕ Y) → ℝ := fun p => dist (f p.1) (f p.2)
    -- check that the induced "distance" is a candidate
    have Fgood : F ∈ candidates X Y := by
      simp only [F, candidates, forall_const,
        dist_eq_zero, Set.mem_setOf_eq]
      repeat' constructor
      · exact fun x y =>
          calc
            F (inl x, inl y) = dist (Φ x) (Φ y) := rfl
            _ = dist x y := Φisom.dist_eq x y
      · exact fun x y =>
          calc
            F (inr x, inr y) = dist (Ψ x) (Ψ y) := rfl
            _ = dist x y := Ψisom.dist_eq x y
      · exact fun x y => dist_comm _ _
      · exact fun x y z => dist_triangle _ _ _
      · exact fun x y =>
          calc
            F (x, y) ≤ diam (range Φ ∪ range Ψ) := by
              have A : ∀ z : X ⊕ Y, f z ∈ range Φ ∪ range Ψ := by
                intro z
                cases z
                · apply mem_union_left; apply mem_range_self
                · apply mem_union_right; apply mem_range_self
              refine dist_le_diam_of_mem ?_ (A _) (A _)
              rw [Φrange, Ψrange]
              exact (p ⊔ q).isCompact.isBounded
            _ ≤ 2 * diam (univ : Set X) + 1 + 2 * diam (univ : Set Y) := I
    let Fb := candidatesBOfCandidates F Fgood
    have : hausdorffDist (range (optimalGHInjl X Y)) (range (optimalGHInjr X Y)) ≤ HD Fb :=
      hausdorffDist_optimal_le_HD _ _ (candidatesBOfCandidates_mem F Fgood)
    refine le_trans this (le_of_forall_gt_imp_ge_of_dense fun r hr => ?_)
    have I1 : ∀ x : X, (⨅ y, Fb (inl x, inr y)) ≤ r := by
      intro x
      have : f (inl x) ∈ (p : Set _) := Φrange ▸ (mem_range_self _)
      rcases exists_dist_lt_of_hausdorffDist_lt this hr
          (hausdorffEDist_ne_top_of_nonempty_of_bounded p.nonempty q.nonempty p.isCompact.isBounded
            q.isCompact.isBounded) with
        ⟨z, zq, hz⟩
      have : z ∈ range Ψ := by rwa [← Ψrange] at zq
      rcases mem_range.1 this with ⟨y, hy⟩
      calc
        (⨅ y, Fb (inl x, inr y)) ≤ Fb (inl x, inr y) :=
          ciInf_le (by simpa only [add_zero] using HD_below_aux1 0) y
        _ = dist (Φ x) (Ψ y) := rfl
        _ = dist (f (inl x)) z := by rw [hy]
        _ ≤ r := le_of_lt hz
    have I2 : ∀ y : Y, (⨅ x, Fb (inl x, inr y)) ≤ r := by
      intro y
      have : f (inr y) ∈ (q : Set _) := Ψrange ▸ (mem_range_self _)
      rcases exists_dist_lt_of_hausdorffDist_lt' this hr
          (hausdorffEDist_ne_top_of_nonempty_of_bounded p.nonempty q.nonempty p.isCompact.isBounded
            q.isCompact.isBounded) with
        ⟨z, zq, hz⟩
      have : z ∈ range Φ := by rwa [← Φrange] at zq
      rcases mem_range.1 this with ⟨x, hx⟩
      calc
        (⨅ x, Fb (inl x, inr y)) ≤ Fb (inl x, inr y) :=
          ciInf_le (by simpa only [add_zero] using HD_below_aux2 0) x
        _ = dist (Φ x) (Ψ y) := rfl
        _ = dist z (f (inr y)) := by rw [hx]
        _ ≤ r := le_of_lt hz
    simp only [HD, ciSup_le I1, ciSup_le I2, max_le_iff, and_self_iff]