∀ {α : Type u_2} {m0 : MeasurableSpace α} {μ : MeasureTheory.Measure α} {s t : Set α},
  MeasureTheory.NullMeasurableSet s μ → MeasureTheory.NullMeasurableSet t μ → MeasureTheory.NullMeasurableSet (s ∪ t) μ