∀ {G : Type u_1} {α : Type u_3} {E : Type u_5} [inst : Group G] [inst_1 : MulAction G α] [inst_2 : MeasurableSpace α]
  [inst_3 : NormedAddCommGroup E] {s : Set α} {μ : MeasureTheory.Measure α} [MeasurableConstSMul G α]
  [MeasureTheory.SMulInvariantMeasure G α μ] [Countable G] [inst_7 : NormedSpace ℝ E],
  MeasureTheory.IsFundamentalDomain G s μ →
    ∀ (f : α → E), MeasureTheory.Integrable f μ → ∫ (x : α), f x ∂μ = ∑' (g : G), ∫ (x : α) in g • s, f x ∂μ