∀ {α : Type u_2} {β : Type u_3} {m0 : MeasurableSpace α} [MeasurableSingletonClass α] {mβ : MeasurableSpace β}
  [MeasurableSingletonClass β] [inst : Fintype α] (μ : MeasureTheory.Measure (α × β)) (y : β),
  μ (Prod.snd ⁻¹' {y}) = ∑ x, μ {(x, y)}