\begin{tabbing}
qpositive($r$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$\=if isint($r$;tt;ff)\+
\\[0ex]then 0 $<$z $r$
\\[0ex]else let $p$,$q$ = $r$ in bor(band(0 $<$z $p$; 0 $<$z $q$); band($p$ $<$z 0; $q$ $<$z 0))
\\[0ex]fi 
\-
\end{tabbing}