\begin{tabbing}
qeq($r$; $s$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$\=if isint($r$;tt;ff)\+
\\[0ex]then if isint($s$;tt;ff) then ($r$ =$_{0}$ $s$) else let $i$,$j$ = $s$ in ($r$ $\ast$ $j$ =$_{0}$ $i$) fi 
\\[0ex]else \=let $p$,$q$ = $r$\+
\\[0ex]in
\\[0ex]if isint($s$;tt;ff) then ($p$ =$_{0}$ $s$ $\ast$ $q$) else let $i$,$j$ = $s$ in ($p$ $\ast$ $j$ =$_{0}$ $i$ $\ast$ $q$) fi 
\-\\[0ex]fi 
\-
\end{tabbing}