-- Lean 4 proof step from Mathlib.MeasureTheory.Function.Jacobian (goal)
Goal:
E : Type u_1
inst : NormedAddCommGroup E
inst_1 : NormedSpace ℝ E
inst_2 : FiniteDimensional ℝ E
s : Set E
f : E → E
f' : E → E →L[ℝ] E
inst_3 : MeasurableSpace E
inst_4 : BorelSpace E
μ : Measure E
inst_5 : μ.IsAddHaarMeasure
hs : MeasurableSet s
hf' : ∀ x ∈ s, HasFDerivWithinAt f (f' x) s x
hf : InjOn f s
ε : ℝ≥0
εpos : 0 < ε
δ : (A : E →L[ℝ] E) → ℝ≥0
hδ :
  ∀ (A : E →L[ℝ] E),
    0 < δ A ∧
      (∀ (B : E →L[ℝ] E), ‖B - A‖ ≤ (↑(δ A) : ℝ) → |B.det - A.det| ≤ (↑ε : ℝ)) ∧
        ∀ (t : Set E) (g : E → E),
          ApproximatesLinearOn g A t (δ A) →
            HMul.hMul (β := ℝ≥0∞) (ENNReal.ofReal |A.det|) ((μ : (a : Set E) → ℝ≥0∞) t : ℝ≥0∞) ≤
              HAdd.hAdd (α := ℝ≥0∞) ((μ : (a : Set E) → ℝ≥0∞) (g '' t) : ℝ≥0∞)
                (HMul.hMul (β := ℝ≥0∞) (↑ε : ℝ≥0∞) ((μ : (a : Set E) → ℝ≥0∞) t : ℝ≥0∞))
t : ℕ → Set E
A : ℕ → E →L[ℝ] E
t_disj : Pairwise (Disjoint on t)
t_meas : ∀ (n : ℕ), MeasurableSet (t n)
t_cover : s ⊆ ⋃ (n : ℕ), t n
ht : ∀ (n : ℕ), ApproximatesLinearOn f (A n) (s ∩ t n) (δ (A n))
s_eq : s = ⋃ (n : ℕ), s ∩ t n
x : ∫⁻ (x : E) in s, ENNReal.ofReal |(f' x).det| ∂μ = ∑' (n : ℕ), ∫⁻ (x : E) in s ∩ t n, ENNReal.ofReal |(f' x).det| ∂μ
x₁ :
  LE.le (α := ℝ≥0∞) (∑' (n : ℕ), ∫⁻ (x : E) in s ∩ t n, ENNReal.ofReal |(f' x).det| ∂μ)
    (∑' (n : ℕ), ∫⁻ (x : E) in s ∩ t n, ENNReal.ofReal |(A n).det| + (↑ε : ℝ≥0∞) ∂μ)
⊢ ∫⁻ (x : E) in s, ENNReal.ofReal |(f' x).det| ∂μ ≤
    HAdd.hAdd (α := ℝ≥0∞) ((μ : (a : Set E) → ℝ≥0∞) (f '' s) : ℝ≥0∞)
      (HMul.hMul (β := ℝ≥0∞) (2 * (↑ε : ℝ≥0∞)) ((μ : (a : Set E) → ℝ≥0∞) s : ℝ≥0∞))
have x₂ : Eq (α := ℝ≥0∞) (∑' (n : ℕ), ∫⁻ (x : E) in s ∩ t n, ENNReal.ofReal |(A n).det| + (↑ε : ℝ≥0∞) ∂μ)
  (∑' (n : ℕ),
    (HMul.hMul (β := ℝ≥0∞) (ENNReal.ofReal |(A n).det|) ((μ : (a : Set E) → ℝ≥0∞) (s ∩ t n) : ℝ≥0∞) +
      HMul.hMul (β := ℝ≥0∞) (↑ε : ℝ≥0∞) ((μ : (a : Set E) → ℝ≥0∞) (s ∩ t n) : ℝ≥0∞)))
Tactic: calc
    (∫⁻ x in s, ENNReal.ofReal |(f' x).det| ∂μ) =
        ∑' n, ∫⁻ x in s ∩ t n, ENNReal.ofReal |(f' x).det| ∂μ := by
      conv_lhs => rw [s_eq]
      rw [lintegral_iUnion]
      · exact fun n => hs.inter (t_meas n)
      · exact pairwise_disjoint_mono t_disj fun n => inter_subset_right
    _ ≤ ∑' n, ∫⁻ _ in s ∩ t n, ENNReal.ofReal |(A n).det| + ε ∂μ := by
      apply ENNReal.tsum_le_tsum fun n => ?_
      apply lintegral_mono_ae
      filter_upwards [(ht n).norm_fderiv_sub_le μ (hs.inter (t_meas n)) f' fun x hx =>
          (hf' x hx.1).mono inter_subset_left]
      intro x hx
      have I : |(f' x).det| ≤ |(A n).det| + ε :=
        calc
          |(f' x).det| = |(A n).det + ((f' x).det - (A n).det)| := by congr 1; abel
          _ ≤ |(A n).det| + |(f' x).det - (A n).det| := abs_add_le _ _
          _ ≤ |(A n).det| + ε := add_le_add le_rfl ((hδ (A n)).2.1 _ hx)
      calc
        ENNReal.ofReal |(f' x).det| ≤ ENNReal.ofReal (|(A n).det| + ε) :=
          ENNReal.ofReal_le_ofReal I
        _ = ENNReal.ofReal |(A n).det| + ε := by
          simp only [ENNReal.ofReal_add, abs_nonneg, NNReal.zero_le_coe, ENNReal.ofReal_coe_nnreal]
    _ = ∑' n, (ENNReal.ofReal |(A n).det| * μ (s ∩ t n) + ε * μ (s ∩ t n)) := by
      simp only [setLIntegral_const, lintegral_add_right _ measurable_const]
    _ ≤ ∑' n, (μ (f '' (s ∩ t n)) + ε * μ (s ∩ t n) + ε * μ (s ∩ t n)) := by
      gcongr
      exact (hδ (A _)).2.2 _ _ (ht _)
    _ = μ (f '' s) + 2 * ε * μ s := by
      conv_rhs => rw [s_eq]
      rw [image_iUnion, measure_iUnion]; rotate_left
      · intro i j hij
        apply Disjoint.image _ hf inter_subset_left inter_subset_left
        exact Disjoint.mono inter_subset_right inter_subset_right (t_disj hij)
      · intro i
        exact
          measurable_image_of_fderivWithin (hs.inter (t_meas i))
            (fun x hx => (hf' x hx.1).mono inter_subset_left)
            (hf.mono inter_subset_left)
      rw [measure_iUnion]; rotate_left
      · exact pairwise_disjoint_mono t_disj fun i => inter_subset_right
      · exact fun i => hs.inter (t_meas i)
      rw [← ENNReal.tsum_mul_left, ← ENNReal.tsum_add]
      congr 1
      ext1 i
      rw [mul_assoc, two_mul, add_assoc]