box to describe $B$. This can be obtained by four iterations of the following 6 substitutions on the three-letter alphabet $\{r, b, w\}$, that is $$\sigma(a) =
\begin{pmatrix}
  r & w & r & w & r & w & r & w \\
  w & r & w & r & w & r & w & w \\
  r & w & r & w & r & w & r & w \\
  w & w & w & w & w & w & w & w
\end{pmatrix},$$ $$\sigma(\bar{a}) =
\begin{pmatrix}
  w & r & w & r & w & r & w & w \\
  r & w & r & w & r & w & r & w \\
  w & r & w & r & w & r & w & w \\
  w & w & w & w & w & w & w & w
\end{pmatrix},$$ $$\sigma(b) =
\begin{pmatrix}
  w & b & w & b & w & b & w & w \\
  b & w & b & w & b & w & b & w \\
  w & b & w & 