∀ {α : Type u_1} {β : Type u_2} {mα : MeasurableSpace α} {mβ : MeasurableSpace β} {μa : MeasureTheory.Measure α}
  {μb : MeasureTheory.Measure β} {f : α → β},
  MeasureTheory.Measure.QuasiMeasurePreserving f μa μb → ∀ {s : Set β}, μb s = 0 → μa (f ⁻¹' s) = 0