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