To sketch the typical steps, let ρ ρ ( Γ Γ , t ) = ρ ρ 0 ( Γ Γ ) σ σ ( Γ Γ , t ) denote the time-dependent probability distribution in phase space. The phase space density σ σ ( Γ Γ , t ) (as well as ρ ρ ( Γ Γ , t ) ) is a solution of the Liouville equation