Let $\mathbb{D} : = \{ 0, 1, 2, 3, 4, 5, 6, 7, 9 \}$ be the D1042: Set of digits.
A D3639: Standard sequence $x : \mathbb{N} \to X$ is a standard digit sequence if and only if \begin{equation} X \subseteq \mathbb{D} \end{equation}