interleaving($T$;$L_{1}$;$L_{2}$;$L$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$\parallel$$L$$\parallel$ = $\parallel$$L_{1}$$\parallel$+$\parallel$$L_{2}$$\parallel$ \& disjoint\_sublists($T$;$L_{1}$;$L_{2}$;$L$)