-- Lean 4 proof step from Mathlib.Analysis.Distribution.AEEqOfIntegralContDiff (hypothesis)
Goal:
E : Type u_1
inst : NormedAddCommGroup E
inst_1 : NormedSpace ℝ E
inst_2 : FiniteDimensional ℝ E
F : Type u_2
inst_3 : NormedAddCommGroup F
inst_4 : NormedSpace ℝ F
inst_5 : CompleteSpace F
H : Type u_3
inst_6 : TopologicalSpace H
I : ModelWithCorners ℝ E H
M : Type u_4
inst_7 : TopologicalSpace M
inst_8 : ChartedSpace H M
inst_9 : IsManifold I ∞ M
inst_10 : MeasurableSpace M
inst_11 : BorelSpace M
inst_12 : T2Space M
f : M → F
μ : Measure M
inst_13 : SigmaCompactSpace M
hf : LocallyIntegrable f μ
h : ∀ (g : M → ℝ), ContMDiff I 𝓘(ℝ, ℝ) ∞ g → HasCompactSupport g → Eq.{u_2 + 1} (α := F) (∫ (x : M), g x • f x ∂μ) 0
this_1 : LocallyCompactSpace H
this_2 : LocallyCompactSpace M
this_3 : SecondCountableTopology H
this : SecondCountableTopology M
x : MetricSpace M := metrizableSpaceMetric M
s : Set M
hs : IsCompact s
δ : ℝ
δpos : 0 < δ
hδ : IsCompact (cthickening δ s)
u : ℕ → ℝ
u_pos : ∀ (n : ℕ), u n ∈ Ioo 0 δ
u_lim : Tendsto u atTop (𝓝 0)
v : ℕ → Set M := fun (n : ℕ) ↦ thickening (u n) s
K : Set M
K_compact : IsCompact K
vK : ∀ (n : ℕ), v n ⊆ K
g : (n : ℕ) → M → ℝ
g_supp : ∀ (n : ℕ), support (g n) = v n
g_diff : ∀ (n : ℕ), ContMDiff I 𝓘(ℝ, ℝ) ∞ (g n)
g_range : ∀ (n : ℕ), range (g n) ⊆ Icc 0 1
hg : ∀ (n : ℕ), ∀ x ∈ s, g n x = 1
⊢ Eq.{u_2 + 1} (α := F) 0 (∫ (x : M) in s, f x ∂μ)
have L : Tendsto.{0, u_2} (β := F) (fun (n : ℕ) ↦ ∫ (x : M), g n x • f x ∂μ) atTop (𝓝 (∫ (x : M) in s, f x ∂μ))
Tactic: have L : Tendsto (fun n ↦ ∫ x, g n x • f x ∂μ) atTop (𝓝 (∫ x in s, f x ∂μ)) := by
    rw [← integral_indicator hs.measurableSet]
    let bound : M → ℝ := K.indicator (fun x ↦ ‖f x‖)
    have A : ∀ n, AEStronglyMeasurable (fun x ↦ g n x • f x) μ :=
      fun n ↦ (g_diff n).continuous.aestronglyMeasurable.smul hf.aestronglyMeasurable
    have B : Integrable bound μ := by
      rw [integrable_indicator_iff K_compact.measurableSet]
      exact (hf.integrableOn_isCompact K_compact).norm
    have C : ∀ n, ∀ᵐ x ∂μ, ‖g n x • f x‖ ≤ bound x := by
      intro n
      filter_upwards with x
      rw [norm_smul]
      refine le_indicator_apply (fun _ ↦ ?_) (fun hxK ↦ ?_)
      · have : ‖g n x‖ ≤ 1 := by
          have := g_range n (mem_range_self (f := g n) x)
          rw [Real.norm_of_nonneg this.1]
          exact this.2
        exact mul_le_of_le_one_left (norm_nonneg _) this
      · have : g n x = 0 := by rw [← notMem_support, g_supp]; contrapose hxK; exact vK n hxK
        simp [this]
    have D : ∀ᵐ x ∂μ, Tendsto (fun n => g n x • f x) atTop (𝓝 (s.indicator f x)) := by
      filter_upwards with x
      by_cases hxs : x ∈ s
      · have : ∀ n, g n x = 1 := fun n ↦ hg n x hxs
        simp [this, indicator_of_mem hxs f]
      · simp_rw [indicator_of_notMem hxs f]
        apply tendsto_const_nhds.congr'
        suffices H : ∀ᶠ n in atTop, g n x = 0 by
          filter_upwards [H] with n hn using by simp [hn]
        obtain ⟨ε, εpos, hε⟩ : ∃ ε, 0 < ε ∧ x ∉ thickening ε s := by
          rw [← hs.isClosed.closure_eq, closure_eq_iInter_thickening s] at hxs
          simpa using hxs
        filter_upwards [(tendsto_order.1 u_lim).2 _ εpos] with n hn
        rw [← notMem_support, g_supp]
        contrapose hε
        exact thickening_mono hn.le s hε
    exact tendsto_integral_of_dominated_convergence bound A B C D