$\forall$$T$:Type, $x$:$T$, $L$, $L_{1}$, $L_{2}$:$T$ List. interleaving($T$;$L_{1}$;$L_{2}$;$L$) $\Rightarrow$ interleaving($T$;$L_{1}$;$x$.$L_{2}$;$x$.$L$)