∀ {α : Type u_1} {β : Type u_2} [inst : MeasurableSpace α] [inst_1 : MeasurableSpace β] [Countable β]
  [MeasurableSingletonClass β] (μ : MeasureTheory.Measure α) (f : α → β),
  Measurable f →
    MeasureTheory.Measure.map f μ = MeasureTheory.Measure.sum fun b => μ (f ⁻¹' {b}) • MeasureTheory.Measure.dirac b