$\forall$$T$:Type, $L_{1}$, $L_{2}$:$T$ List, $x$, $y$:$T$. $L_{1}$ $\leq$ $L_{2}$ $\Rightarrow$ $x$ before $y$ $\in$ $L_{1}$ $\Rightarrow$ $x$ before $y$ $\in$ $L_{2}$