$\forall$$T$:Type, $l_{1}$, $l_{2}$:($T$ List). $l_{1}$ $\parallel$ $l_{2}$ $\Leftarrow\!\Rightarrow$ ($\exists$$l$:$T$ List. ($l_{1}$ $\leq$ $l$ \& $l_{2}$ $\leq$ $l$))