$\forall$$A$, $B$:Type, ${\it xs}$:($A$ List), ${\it ys}$:($B$ List), $x$:$A$, $y$:$B$.
\\[0ex]($<$$x$, $y$$>$ $\in$ zip(${\it xs}$;${\it ys}$)) $\Rightarrow$ \{($x$ $\in$ ${\it xs}$) \& ($y$ $\in$ ${\it ys}$)\}