$\forall$$T$:Type, $L_{1}$, $L_{2}$:$T$ List, $x$, $y$:$T$.
\\[0ex]($y$ $\in$ $L_{1}$) $\Rightarrow$ $\neg$($y$ $\in$ $L_{2}$) $\Rightarrow$ $x$ before $y$ $\in$ $L_{1}$ @ $L_{2}$ $\Rightarrow$ $x$ before $y$ $\in$ $L_{1}$