reduce($f$; $k$; ${\it as}$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$rec{-}case(${\it as}$) of [] =$>$ $k$ $\mid$ $a$::${\it as'}$ =$>$ .$f$($a$,reduce($f$; $k$; ${\it as'}$))
\\[0ex]\emph{(recursive)}