\begin{tabbing}
t\_iterate($l$;$n$;$t$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$\=Case($t$)\+
\\[0ex]Case\= $x$;$y$ =$>$\+
\\[0ex]$n$(t\_iterate($l$;$n$;$x$),t\_iterate($l$;$n$;$y$))
\-\\[0ex]Case\= tree\_leaf($x$) =$>$\+
\\[0ex]$l$($x$)
\-\\[0ex]Default =$>$ True
\-\\[0ex]\emph{(recursive)}
\end{tabbing}