A D1721: Random collection $X : \mathbb{N} \to \mathsf{Random}(\Omega \to \Xi)$ is stationary if and only if \begin{equation} \forall \, M \in \mathbb{N} : \forall \, n_1, \ldots, n_M \in \mathbb{N} : \forall \, k \in \mathbb{N} : (X_{n_1}, \ldots, X_{n_M}) \overset{d}{=} (X_{n_1 + k}, \ldots, X_{n_M + k}) \end{equation}