Case tree\_node($x$) =$>$ ${\it body}$($x$) ${\it cont}$($x_{1}$,$z$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$($\lambda$$x_{1}$.inr($x_{2}$) $\Rightarrow$ ${\it body}$(hd(($x_{2}$.tl($x_{1}$)))) ; ${\it cont}$(hd($x_{1}$),$z$))($x_{1}$.nil)