∀ {α : Type u_1} {m : MeasurableSpace α} {μ : MeasureTheory.Measure α} {s t : Set α},
  MeasureTheory.NullMeasurableSet s μ → MeasureTheory.NullMeasurableSet t μ → μ (symmDiff s t) = μ (s \ t) + μ (t \ s)