$\forall$$T$:Type, $L_{1}$, $L_{2}$, $L_{3}$:($T$ List). $L_{1}$ $\leq$ $L_{2}$ $\Rightarrow$ $L_{3}$ $\parallel$ $L_{2}$ $\Rightarrow$ $L_{3}$ $\parallel$ $L_{1}$