\begin{tabbing}
firstn($n$; ${\it as}$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$\=rec{-}case(${\it as}$) of [] =$>$ [] $\mid$ $a$::${\it as'}$ =$>$ .if 0 $<$z $n$\+
\\[0ex]then cons($a$; firstn(($n$ {-} 1); ${\it as'}$))
\\[0ex]else []
\\[0ex]fi 
\-\\[0ex]\emph{(recursive)}
\end{tabbing}