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