∀ {α : Type u_1} {β : Type u_2} {m : MeasurableSpace α} {μ : MeasureTheory.Measure α} [inst : NormedAddCommGroup β]
  (f g : α → β) (hf : MeasureTheory.Integrable f μ) (hg : MeasureTheory.Integrable g μ),
  MeasureTheory.Integrable.toL1 (f + g) ⋯ = MeasureTheory.Integrable.toL1 f hf + MeasureTheory.Integrable.toL1 g hg