∀ {α : Type u_1} {β : Type u_2} [inst : MeasurableSpace α] [inst_1 : MeasurableSpace β] {μ : MeasureTheory.Measure α}
  {ν : MeasureTheory.Measure β} [MeasureTheory.SFinite ν] [MeasureTheory.SFinite μ] {L : Type u_5} [inst_4 : RCLike L]
  (f : α → L) (g : β → L), (∫ (x : α), f x ∂μ) * ∫ (y : β), g y ∂ν = ∫ (z : α × β), f z.1 * g z.2 ∂μ.prod ν