$\forall$$T_{1}$, $T_{2}$:Type, ${\it as}$:$T_{1}$ List, ${\it bs}$:$T_{2}$ List. $\parallel$zip(${\it as}$;${\it bs}$)$\parallel\leq\parallel$${\it as}$$\parallel$ \& $\parallel$zip(${\it as}$;${\it bs}$)$\parallel\leq\parallel$${\it bs}$$\parallel$