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