$\forall$$T$:Type, $L_{1}$, $L_{2}$:($T$ List). $L_{1}$ $\subseteq$ $L_{2}$ $\Rightarrow$ ($\parallel$$L_{1}$$\parallel$ = $\parallel$$L_{2}$$\parallel$ $\in$ $\mathbb{Z}$) $\Rightarrow$ ($L_{1}$ = $L_{2}$)