first $x$ $\in$ ${\it as}$ s.t. $P$($x$) else $d$ $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$rec{-}case(filter($\lambda$$x$.$P$($x$);${\it as}$)) of [] =$>$ $d$ $\mid$ $a$::$b$ =$>$ .$a$