∀ {α : Type u_1} {β : Type u_2} [inst : MeasurableSpace α] [inst_1 : MeasurableSpace β] (f : α → β),
  MeasureTheory.Measure.mapₗ f =
    if hf : Measurable f then MeasureTheory.Measure.liftLinear (MeasureTheory.OuterMeasure.map f) ⋯ else 0