\begin{equation}
\boldsymbol{A}_l =
\begin{cases}
\begin{bmatrix}
\vdots\\
\boldsymbol{x}_m\\
\vdots
\end{bmatrix}, & \text{if} \,\,\, l = 1,\\
\\
\begin{bmatrix}
\vdots\\
\boldsymbol{n}_{i}^{(l-1)}\\
\vdots
\end{bmatrix}, & \text{if} \,\,\, 1 < l \leq L + 1\\
\end{cases},
\nonumber
\end{equation}
