-- 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))
i : Fin (n + 1)
f : (Fin (n + 1) → ℝ) → E
f' : (Fin (n + 1) → ℝ) →L[ℝ] E
hfc : ContinuousOn f ((Box.Icc : (a : Box (Fin (n + 1))) → Set (Fin (n + 1) → ℝ)) I)
x : Fin (n + 1) → ℝ
hxI :
  Membership.mem (γ := Set (Fin (n + 1) → ℝ))
    ((Box.Icc : (a : Box (Fin (n + 1))) → Set (Fin (n + 1) → ℝ)) I : Set (Fin (n + 1) → ℝ)) x
a : E
ε : ℝ
h0 : 0 < ε
hε :
  ∀ (y : 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 : Set (Fin (n + 1) → ℝ)) y →
      ‖HSub.hSub (β := E) (f y - a) ((f' : (a : Fin (n + 1) → ℝ) → E) (y - x) : E)‖ ≤ ε * ‖y - x‖
c : ℝ≥0
hc : I.distortion ≤ c
e : ℝ → (Fin n → ℝ) → Fin (n + 1) → ℝ := i.insertNth (α := fun (x : Fin (n + 1)) ↦ ℝ)
Hl : I.lower i ∈ Set.Icc (I.lower i) (I.upper i)
Hu : I.upper i ∈ Set.Icc (I.lower i) (I.upper i)
Hi : ∀ x ∈ Set.Icc (I.lower i) (I.upper i), Integrable (I.face i) ⊥ (f ∘ e x) BoxAdditiveMap.volume
this :
  ∀ (y : Fin n → ℝ),
    Membership.mem (γ := Set (Fin n → ℝ)) ((Box.Icc : (a : Box (Fin n)) → Set (Fin n → ℝ)) (I.face i) : Set (Fin n → ℝ))
        y →
      ‖HSub.hSub (α := E)
            ((f' : (a : Fin (n + 1) → ℝ) → E) (Pi.single (M := fun (a : Fin (n + 1)) ↦ ℝ) i (I.upper i - I.lower i)) :
              E)
            (f (e (I.upper i) y) - f (e (I.lower i) y))‖ ≤
        2 * ε * diam (α := Fin (n + 1) → ℝ) ((Box.Icc : (a : Box (Fin (n + 1))) → Set (Fin (n + 1) → ℝ)) I)
j :
  ‖HSMul.hSMul (α := ℝ) (β := E) (∏ j : Fin (n + 1), (I.upper j - I.lower j))
          ((f' : (a : Fin (n + 1) → ℝ) → E) (Pi.single (M := fun (a : Fin (n + 1)) ↦ ℝ) i 1) : E) -
        (integral (I.face i) ⊥ (f ∘ e (I.upper i)) BoxAdditiveMap.volume -
          integral (I.face i) ⊥ (f ∘ e (I.lower i)) BoxAdditiveMap.volume)‖ =
    norm.{u} (E := E)
      (integral (I.face i) ⊥
        (fun (x : Fin n → ℝ) ↦
          HSub.hSub (α := E)
            ((f' : (a : Fin (n + 1) → ℝ) → E) (Pi.single (M := fun (a : Fin (n + 1)) ↦ ℝ) i (I.upper i - I.lower i)) :
              E)
            (f (e (I.upper i) x) - f (e (I.lower i) x)))
        BoxAdditiveMap.volume)
j₁ :
  norm.{u} (E := E)
      (integral (I.face i) ⊥
        (fun (x : Fin n → ℝ) ↦
          HSub.hSub (α := E)
            ((f' : (a : Fin (n + 1) → ℝ) → E) (Pi.single (M := fun (a : Fin (n + 1)) ↦ ℝ) i (I.upper i - I.lower i)) :
              E)
            (f (e (I.upper i) x) - f (e (I.lower i) x)))
        BoxAdditiveMap.volume) ≤
    ENNReal.toReal ((volume : (a : Set (Fin n → ℝ)) → ℝ≥0∞) (↑(I.face i) : Set (Fin n → ℝ))) *
      (2 * ε * (↑c : ℝ) * (I.upper i - I.lower i))
⊢ ‖HSMul.hSMul (α := ℝ) (β := E) (∏ j : Fin (n + 1), (I.upper j - I.lower j))
          ((f' : (a : Fin (n + 1) → ℝ) → E) (Pi.single (M := fun (a : Fin (n + 1)) ↦ ℝ) i 1) : E) -
        (integral (I.face i) ⊥ (f ∘ e (I.upper i)) BoxAdditiveMap.volume -
          integral (I.face i) ⊥ (f ∘ e (I.lower i)) BoxAdditiveMap.volume)‖ ≤
    2 * ε * (↑c : ℝ) * ∏ j : Fin (n + 1), (I.upper j - I.lower j)
have j₂ : ENNReal.toReal ((volume : (a : Set (Fin n → ℝ)) → ℝ≥0∞) (↑(I.face i) : Set (Fin n → ℝ))) *
    (2 * ε * (↑c : ℝ) * (I.upper i - I.lower i)) =
  2 * ε * (↑c : ℝ) * ∏ j : Fin (n + 1), (I.upper j - I.lower j)
Tactic: calc
    ‖(∏ j, (I.upper j - I.lower j)) • f' (Pi.single i 1) -
            (integral (I.face i) ⊥ (f ∘ e (I.upper i)) BoxAdditiveMap.volume -
              integral (I.face i) ⊥ (f ∘ e (I.lower i)) BoxAdditiveMap.volume)‖ =
        ‖integral.{0, u, u} (I.face i) ⊥
            (fun x : Fin n → ℝ =>
              f' (Pi.single i (I.upper i - I.lower i)) -
                (f (e (I.upper i) x) - f (e (I.lower i) x)))
            BoxAdditiveMap.volume‖ := by
      rw [← integral_sub (Hi _ Hu) (Hi _ Hl), ← Box.volume_face_mul i, mul_smul, ← Box.volume_apply,
        ← BoxAdditiveMap.toSMul_apply, ← integral_const, ← BoxAdditiveMap.volume,
        ← integral_sub (integrable_const _) ((Hi _ Hu).sub (Hi _ Hl))]
      simp only [(· ∘ ·), Pi.sub_def, ← f'.map_smul, ← Pi.single_smul', smul_eq_mul, mul_one]
    _ ≤ (volume (I.face i : Set (Fin n → ℝ))).toReal * (2 * ε * c * (I.upper i - I.lower i)) := by
      -- The hard part of the estimate was done above, here we just replace `diam I.Icc`
      -- with `c * (I.upper i - I.lower i)`
      refine norm_integral_le_of_le_const (fun y hy => (this y hy).trans ?_) volume
      rw [mul_assoc (2 * ε)]
      gcongr
      exact I.diam_Icc_le_of_distortion_le i hc
    _ = 2 * ε * c * ∏ j, (I.upper j - I.lower j) := by
      rw [← measureReal_def, ← Measure.toBoxAdditive_apply, Box.volume_apply,
        ← I.volume_face_mul i]
      ac_rfl