Label in table should cross another cell

Here I \llap the $a={}$ to the left of $0$. The {} is necessary after the = to convince the = that an actual number will follow and to set the spacing appropriately.

\documentclass{scrartcl}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}

\begin{document}
\begin{table}[ht]
\centering
\begin{tabular}{rccc}
&\llap{$a={}$}$0$&$1$&$2$\\\cline{2-4}
$c=0$&\\\cline{2-4}
$1$&\\\cline{2-4}
$2$&\\\cline{2-4}
\end{tabular}
\caption{caption}
\end{table}
\end{document}