\begin{equation}
    \mathcal{S}\left(\mathbb{R}^{d}\right)=\left\{\pmb{x}=\left(\pmb{x}_{1}, \ldots, \pmb{x}_{n}\right): \pmb{x}_{i} \in \mathbb{R}^{d}, n \in \mathbb{N}\right\}.\notag
\end{equation}
