$\forall$$A$, $B$:Type, ${\it xs}$:$A$ List, ${\it ys}$:$B$ List, $x$:$A$, $y$:$B$. ($\langle$$x$$,\,$$y$$\rangle$ $\in$ zip(${\it xs}$;${\it ys}$)) $\Rightarrow$ \{($x$ $\in$ ${\it xs}$) \& ($y$ $\in$ ${\it ys}$)\}