# [Tex/LaTex] Matrix with labels nested in braces

matrices

What's the best way to produce a matrix with labels that are nested in braces, like this:

In today's I have too much free time episode we will attack a question posted without a MWE. Absence of a MWE gives us the right to go as esoteric as possible hence here is one Tweak this, tweak that possibility.

\documentclass[tikz]{standalone}
\usetikzlibrary{matrix,trees}

\begin{document}
\begin{tikzpicture}[level distance=7mm,
level 1/.style={sibling distance=8mm},
level 2/.style={sibling distance=4mm},
]
\matrix (m) [mymatrix/.style={execute at empty cell=
\ifnum\pgfmatrixcurrentrow<5
\ifnum\pgfmatrixcurrentcolumn>4
\node\bgroup$0$\egroup;\fi
\else
\ifnum\pgfmatrixcurrentcolumn<5
\node\bgroup$0$\egroup;\fi
\fi
},mymatrix,
right delimiter={]},left delimiter={[}]{
&&&&&&&\\
&&&&&&&\\
&&&&&&&\\
&&&&&&&\\
&&&&&&&\\
&&&&&&&\\
&&&&&&&\\
&&&&&&&\\
};
\begin{scope}[edge from parent path=
{(\tikzparentnode.south) .. controls +(0,-0.2) and +(0,0.2)
.. (\tikzchildnode.north)}]
\node at ([yshift=1.5cm]m.67) {$A$}
child {node {$1$}
child {node {S}}
child {node {I}}}
child {node {$2$}
child {node {S}}
child {node {I}}
};
\node at ([yshift=1.5cm]m.113) {$B$}
child {node {$1$}
child {node {S}}
child {node {I}}}
child {node {$2$}
child {node {S}}
child {node {I}}
};
\end{scope}

\begin{scope}[edge from parent path=
{(\tikzparentnode.east) .. controls +(0.2,0) and +(-0.2,0)
.. (\tikzchildnode.west)},level distance=6mm,
level 1/.style={sibling distance=9.2mm},
level 2/.style={sibling distance=4.5mm}]
\node at ([xshift=-1.6cm]m.152) {$A$}[grow=right]
child {node {$1$}
child {node {S}}
child {node {I}}}
child {node {$2$}
child {node {S}}
child {node {I}}
};
\node at ([xshift=-1.6cm]m.-152) {$B$}[grow=right]
child {node {$1$}
child {node {S}}
child {node {I}}}
child {node {$2$}
child {node {S}}
child {node {I}}
};
\end{scope}
\end{tikzpicture}
\end{document}