$\forall$$T$:Type, $L$, $L_{1}$, $L_{2}$:$T$ List.
\\[0ex]interleaving($T$;$L_{1}$;$L_{2}$;$L$) $\Rightarrow$ \{$\forall$$x$:$T$. ($x$ $\in$ $L$) $\Leftrightarrow$ ($x$ $\in$ $L_{1}$) $\vee$ ($x$ $\in$ $L_{2}$)\}