$\vdash$  $\forall$$T$:Type, $L_{1}$, $L_{2}$:($T$ List). $L_{1}$ $\subseteq$ $L_{2}$ $\Rightarrow$ \{$\forall$$x$, $y$:$T$. $x$ before $y$ $\in$ $L_{1}$ $\Rightarrow$ $x$ before $y$ $\in$ $L_{2}$\}