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