/-- If a set of ae strongly measurable functions is stable under addition and approximates
characteristic functions in `ℒp`, then it is dense in `ℒp`. -/
theorem Memℒp.induction_dense (hp_ne_top : p ≠ ∞) (P : (α → E) → Prop)
    (h0P :
      ∀ (c : E) ⦃s : Set α⦄,
        MeasurableSet s →
          μ s < ∞ →
            ∀ {ε : ℝ≥0∞}, ε ≠ 0 → ∃ g : α → E, snorm (g - s.indicator fun _ => c) p μ ≤ ε ∧ P g)
    (h1P : ∀ f g, P f → P g → P (f + g)) (h2P : ∀ f, P f → AEStronglyMeasurable f μ) {f : α → E}
    (hf : Memℒp f p μ) {ε : ℝ≥0∞} (hε : ε ≠ 0) : ∃ g : α → E, snorm (f - g) p μ ≤ ε ∧ P g := by
  rcases eq_or_ne p 0 with (rfl | hp_pos)
  · rcases h0P (0 : E) MeasurableSet.empty (by simp only [measure_empty, zero_lt_top])
        hε with ⟨g, _, Pg⟩
    exact ⟨g, by simp only [snorm_exponent_zero, zero_le'], Pg⟩
  suffices H :
    ∀ (f' : α →ₛ E) (δ : ℝ≥0∞) (hδ : δ ≠ 0), Memℒp f' p μ → ∃ g, snorm (⇑f' - g) p μ ≤ δ ∧ P g
  · obtain ⟨η, ηpos, hη⟩ := exists_Lp_half E μ p hε
    rcases hf.exists_simpleFunc_snorm_sub_lt hp_ne_top ηpos.ne' with ⟨f', hf', f'_mem⟩
    rcases H f' η ηpos.ne' f'_mem with ⟨g, hg, Pg⟩
    refine' ⟨g, _, Pg⟩
    convert (hη _ _ (hf.aestronglyMeasurable.sub f'.aestronglyMeasurable)
          (f'.aestronglyMeasurable.sub (h2P g Pg)) hf'.le hg).le using 2
    simp only [sub_add_sub_cancel]
  apply SimpleFunc.induction
  · intro c s hs ε εpos Hs
    rcases eq_or_ne c 0 with (rfl | hc)
    · rcases h0P (0 : E) MeasurableSet.empty (by simp only [measure_empty, zero_lt_top])
          εpos with ⟨g, hg, Pg⟩
      rw [← snorm_neg, neg_sub] at hg
      refine' ⟨g, _, Pg⟩
      convert hg
      ext x
      simp only [SimpleFunc.const_zero, SimpleFunc.coe_piecewise, SimpleFunc.coe_zero,
        piecewise_eq_indicator, indicator_zero', Pi.zero_apply, indicator_zero]
    · have : μ s < ∞ := SimpleFunc.measure_lt_top_of_memℒp_indicator hp_pos hp_ne_top hc hs Hs
      rcases h0P c hs this εpos with ⟨g, hg, Pg⟩
      rw [← snorm_neg, neg_sub] at hg
      exact ⟨g, hg, Pg⟩
  · intro f f' hff' hf hf' δ δpos int_ff'
    obtain ⟨η, ηpos, hη⟩ := exists_Lp_half E μ p δpos
    rw [SimpleFunc.coe_add,
      memℒp_add_of_disjoint hff' f.stronglyMeasurable f'.stronglyMeasurable] at int_ff'
    rcases hf η ηpos.ne' int_ff'.1 with ⟨g, hg, Pg⟩
    rcases hf' η ηpos.ne' int_ff'.2 with ⟨g', hg', Pg'⟩
    refine' ⟨g + g', _, h1P g g' Pg Pg'⟩
    

Proof state:
case H.h_add.intro.intro.intro.intro.intro.intro
α : Type u_1
β : Type u_2
ι : Type u_3
E : Type u_4
F : Type u_5
𝕜 : Type u_6
inst✝¹ : MeasurableSpace α
inst✝ : NormedAddCommGroup E
f✝¹ : α → E
p : ℝ≥0∞
μ : Measure α
hp_ne_top : p ≠ ⊤
P : (α → E) → Prop
h0P :
  ∀ (c : E) ⦃s : Set α⦄,
    MeasurableSet s → ↑↑μ s < ⊤ → ∀ {ε : ℝ≥0∞}, ε ≠ 0 → ∃ g, snorm (g - Set.indicator s fun x => c) p μ ≤ ε ∧ P g
h1P : ∀ (f g : α → E), P f → P g → P (f + g)
h2P : ∀ (f : α → E), P f → AEStronglyMeasurable f μ
f✝ : α → E
hf✝ : Memℒp f✝ p
ε : ℝ≥0∞
hε : ε ≠ 0
hp_pos : p ≠ 0
f f' : α →ₛ E
hff' : Disjoint (support ↑f) (support ↑f')
hf : ∀ (δ : ℝ≥0∞), δ ≠ 0 → Memℒp (↑f) p → ∃ g, snorm (↑f - g) p μ ≤ δ ∧ P g
hf' : ∀ (δ : ℝ≥0∞), δ ≠ 0 → Memℒp (↑f') p → ∃ g, snorm (↑f' - g) p μ ≤ δ ∧ P g
δ : ℝ≥0∞
δpos : δ ≠ 0
int_ff' : Memℒp (↑f) p ∧ Memℒp (↑f') p
η : ℝ≥0∞
ηpos : 0 < η
hη :
  ∀ (f g : α → E),
    AEStronglyMeasurable f μ → AEStronglyMeasurable g μ → snorm f p μ ≤ η → snorm g p μ ≤ η → snorm (f + g) p μ < δ
g : α → E
hg : snorm (↑f - g) p μ ≤ η
Pg : P g
g' : α → E
hg' : snorm (↑f' - g') p μ ≤ η
Pg' : P g'
⊢ snorm (↑(f + f') - (g + g')) p μ ≤ δ

Next tactic:
convert (hη _ _ (f.aestronglyMeasurable.sub (h2P g Pg))
          (f'.aestronglyMeasurable.sub (h2P g' Pg')) hg hg').le using 2