\begin{tabbing} st{-}lookup(${\it tab}$;$x$) \\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$\=let $K$,$p$,$f$ = ${\it tab}$ in \+ \\[0ex]let $n$\= = mu($\lambda$$n$.$p$ $<$z $n$ $\vee_{b}$$K$ $\leq$z $n$ $\vee_{b}$($f$($n$)).1 =a1 $x$) in\+ \\[0ex]if $p$ $<$z $n$ $\vee_{b}$$K$ $\leq$z $n$ then inr $\cdot$ else inl (($f$($n$)).2) fi \-\- \end{tabbing}