\begin{tabbing}
first($e$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$\=i\=f time($e$)=$_{2}$0$\rightarrow$ true$_{2}$\+\+
\\[0ex]; isnull(a(loc($e$);time($e$)$-$1))$\rightarrow$ first($\langle$loc($e$)$,\,$time($e$)$-$1$\rangle$)
\-\\[0ex]else false$_{2}$ fi
\-\\[0ex]\emph{(recursive)}
\end{tabbing}