 Now stir. Mathematically, stirring is represented by the time evolution \(T\), meaning that \(T(S)\) is the region occupied by the scotch after one unit of mixing time. Intuitively we say that the cocktail is thoroughly mixed, if the concentration of scotch equals \(\mu(S) / \mu(C)\) not only with respect to the whole volume of fluid, but with respect to any region \(V\) in that volume. Hence, the drink is thoroughly mixed at time \(n\) if