-- Lean 4 proof step from Mathlib.Analysis.BoxIntegral.DivergenceTheorem (hypothesis)
Goal:
E : Type u
inst : NormedAddCommGroup E
inst_1 : NormedSpace ℝ E
n : ℕ
inst_2 : CompleteSpace E
I : Box (Fin (n + 1))
f : (Fin (n + 1) → ℝ) → E
f' : (Fin (n + 1) → ℝ) → (Fin (n + 1) → ℝ) →L[ℝ] E
s : Set (Fin (n + 1) → ℝ)
hs : s.Countable
Hs : ∀ x ∈ s, ContinuousWithinAt f ((Box.Icc : (a : Box (Fin (n + 1))) → Set (Fin (n + 1) → ℝ)) I) x
Hd :
  ∀ (x : Fin (n + 1) → ℝ),
    Membership.mem (γ := Set (Fin (n + 1) → ℝ))
        ((Box.Icc : (a : Box (Fin (n + 1))) → Set (Fin (n + 1) → ℝ)) I \ s : Set (Fin (n + 1) → ℝ)) x →
      HasFDerivWithinAt f (f' x) ((Box.Icc : (a : Box (Fin (n + 1))) → Set (Fin (n + 1) → ℝ)) I) x
i : Fin (n + 1)
Hc : ContinuousOn f ((Box.Icc : (a : Box (Fin (n + 1))) → Set (Fin (n + 1) → ℝ)) I)
fI : ℝ → Box (Fin n) → E :=
  fun (y : ℝ) (J : Box (Fin n)) ↦
    integral J GP (fun (x : Fin n → ℝ) ↦ f (i.insertNth (α := fun (a : Fin (n + 1)) ↦ ℝ) y x)) BoxAdditiveMap.volume
fb : (↑(Set.Icc (I.lower i) (I.upper i)) : Type) → Fin n →ᵇᵃ[(↑(I.face i) : WithTop (Box (Fin n)))] E :=
  fun (x : (↑(Set.Icc (I.lower i) (I.upper i)) : Type)) ↦
    Integrable.toBoxAdditive (vol := BoxAdditiveMap.toSMul (Measure.toBoxAdditive volume)) ⋯
F : Fin (n + 1) →ᵇᵃ[(↑I : WithTop (Box (Fin (n + 1))))] E := BoxAdditiveMap.upperSubLower I i fI fb ⋯
c : ℝ≥0
x : Fin (n + 1) → ℝ
hx :
  Membership.mem (γ := Set (Fin (n + 1) → ℝ))
    ((Box.Icc : (a : Box (Fin (n + 1))) → Set (Fin (n + 1) → ℝ)) I ∩ s : Set (Fin (n + 1) → ℝ)) x
ε : ℝ
ε0 : ε > 0
⊢ ∃ δ > 0,
    ∀ J ≤ I,
      (Box.Icc : (a : Box (Fin (n + 1))) → Set (Fin (n + 1) → ℝ)) J ⊆ closedBall x δ →
        Membership.mem (γ := Set (Fin (n + 1) → ℝ))
            ((Box.Icc : (a : Box (Fin (n + 1))) → Set (Fin (n + 1) → ℝ)) J : Set (Fin (n + 1) → ℝ)) x →
          (GP.bDistortion = true → J.distortion ≤ c) →
            dist (α := E)
                ((((BoxAdditiveMap.volume : (a : Box (Fin (n + 1))) → E →L[ℝ] E) J : E →L[ℝ] E) : (a : E) → E)
                    ((f' x : (a : Fin (n + 1) → ℝ) → E) (Pi.single (M := fun (a : Fin (n + 1)) ↦ ℝ) i 1) : E) :
                  E)
                ((F : (a : Box (Fin (n + 1))) → E) J : E) ≤
              ε
have this : ∀ᶠ (δ : ℝ) in 𝓝[>] 0,
  δ ∈ Set.Ioc 0 (1 / 2) ∧
    (∀ y₁ ∈ closedBall x δ ∩ (Box.Icc : (a : Box (Fin (n + 1))) → Set (Fin (n + 1) → ℝ)) I,
        ∀ y₂ ∈ closedBall x δ ∩ (Box.Icc : (a : Box (Fin (n + 1))) → Set (Fin (n + 1) → ℝ)) I, ‖f y₁ - f y₂‖ ≤ ε / 2) ∧
      (2 * δ) ^ (n + 1) *
          norm (E := E) ((f' x : (a : Fin (n + 1) → ℝ) → E) (Pi.single (M := fun (a : Fin (n + 1)) ↦ ℝ) i 1) : E) ≤
        ε / 2
Tactic: have : ∀ᶠ δ in 𝓝[>] (0 : ℝ), δ ∈ Ioc (0 : ℝ) (1 / 2) ∧
        (∀ᵉ (y₁ ∈ closedBall x δ ∩ (Box.Icc I)) (y₂ ∈ closedBall x δ ∩ (Box.Icc I)),
              ‖f y₁ - f y₂‖ ≤ ε / 2) ∧ (2 * δ) ^ (n + 1) * ‖f' x (Pi.single i 1)‖ ≤ ε / 2 := by
      refine .and (Ioc_mem_nhdsGT one_half_pos) (.and ?_ ?_)
      · rcases ((nhdsWithin_hasBasis nhds_basis_closedBall _).tendsto_iff nhds_basis_closedBall).1
            (Hs x hx.2) _ (half_pos <| half_pos ε0) with ⟨δ₁, δ₁0, hδ₁⟩
        filter_upwards [Ioc_mem_nhdsGT δ₁0] with δ hδ y₁ hy₁ y₂ hy₂
        have : closedBall x δ ∩ (Box.Icc I) ⊆ closedBall x δ₁ ∩ (Box.Icc I) := by gcongr; exact hδ.2
        rw [← dist_eq_norm]
        calc
          dist (f y₁) (f y₂) ≤ dist (f y₁) (f x) + dist (f y₂) (f x) := dist_triangle_right _ _ _
          _ ≤ ε / 2 / 2 + ε / 2 / 2 := add_le_add (hδ₁ _ <| this hy₁) (hδ₁ _ <| this hy₂)
          _ = ε / 2 := add_halves _
      · have : ContinuousWithinAt (fun δ : ℝ => (2 * δ) ^ (n + 1) * ‖f' x (Pi.single i 1)‖)
            (Ioi 0) 0 := ((continuousWithinAt_id.const_mul _).pow _).mul_const _
        refine this.eventually (ge_mem_nhds ?_)
        simpa using half_pos ε0