is{-}query(${\it In}$;${\it isupdate}$;$e$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$($\uparrow$($e$ $\in_{b}$ ${\it In}$)) \& ($\neg$($\uparrow$(${\it isupdate}$(${\it In}$($e$)))))