-- Lean 4 proof step from Mathlib.Analysis.BoxIntegral.DivergenceTheorem (goal)
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) ≤
              HMul.hMul (β := ℝ) ε
                ((BoxAdditiveMap.restrict (Measure.toBoxAdditive volume) (↑I : WithTop (Box (Fin (n + 1)))) ⋯ :
                      (a : Box (Fin (n + 1))) → ℝ)
                    J :
                  ℝ)
have refine_4 : ∀ (ε' : ℝ) (ε'0 : 0 < ε') (hlt : 2 * (↑c : ℝ) * ε' < ε),
  ∃ 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) ≤
              HMul.hMul (β := ℝ) ε
                ((BoxAdditiveMap.restrict (Measure.toBoxAdditive volume) (↑I : WithTop (Box (Fin (n + 1)))) sorry :
                      (a : Box (Fin (n + 1))) → ℝ)
                    J :
                  ℝ)
Tactic: rcases exists_pos_mul_lt ε0 (2 * c) with ⟨ε', ε'0, hlt⟩