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