${\it as}$$\backslash$[$i$] $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$if $i$$\leq_{2}$0$\rightarrow$ tl(${\it as}$) else Case of ${\it as}$; nil $\rightarrow$ nil ; ${\it a'}$.${\it as'}$, rec: $\rightarrow$ ${\it a'}$.${\it as'}$$\backslash$[$i$$-$1] fi
\\[0ex]\emph{(recursive)}