\begin{tabbing}
qrep($r$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$\=if isint($r$)\+
\\[0ex]then $<$$r$, 1$>$
\\[0ex]else \=let $p$,$q$ = $r$\+
\\[0ex]in
\\[0ex]spreadn(gcd\_reduce($p$; $q$); $g$,$a$,$b$.if 0 $\leq$z $b$ then $<$$a$, $b$$>$ else $<${-}$a$, {-}$b$$>$ fi )
\-\\[0ex]fi 
\-
\end{tabbing}