∀ {Ω : Type u_1} {mΩ : MeasurableSpace Ω} {P : MeasureTheory.Measure Ω} {μ : MeasureTheory.Measure ℝ} {X : Ω → ℝ},
  ProbabilityTheory.HasLaw X μ P → ProbabilityTheory.variance X P = ProbabilityTheory.variance id μ