$\forall$$T$:Type, ${\it as}$, ${\it bs}$:$T$ List, $a$, $b$:$T$. $a$.${\it as}$ $\parallel$ $b$.${\it bs}$ $\Leftrightarrow$ $a$ $=$ $b$ \& ${\it as}$ $\parallel$ ${\it bs}$