\begin{tabbing}
priority{-}select($f$;$g$;${\it as}$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$list\_accum(\=$x$,$m$.if isl($x$)$\rightarrow$ $x$ ; $f$($m$)$\rightarrow$ inl(true$_{2}$) ; $g$($m$)$\rightarrow$ inl(false$_{2}$) else $x$ fi;\+
\\[0ex]inr($\cdot$);
\\[0ex]${\it as}$)
\-
\end{tabbing}