$\forall$$T_{1}$, $T_{2}$:Type, ${\it as}$:$T_{1}$ List, ${\it bs}$:$T_{2}$ List. $\parallel$${\it as}$$\parallel$ $=$ $\parallel$${\it bs}$$\parallel$ $\in$ $\mathbb{Z}$ $\Rightarrow$ $\parallel$zip(${\it as}$;${\it bs}$)$\parallel$ $=$ $\parallel$${\it as}$$\parallel$ $\in$ $\mathbb{Z}$