\begin{tabbing}
w{-}knum($w$;$i$;$k$;$t$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$\=sum(if ($\neg_{b}$w{-}isnull($w$; w{-}a($w$; $i$; $n$))) $\wedge_{b}$ w{-}kind($w$; w{-}a($w$; $i$; $n$)) = $k$\+
\\[0ex]then 1
\\[0ex]else 0
\\[0ex]fi  $\mid$ $n$ $<$ $t$)
\-
\end{tabbing}