$\forall$$T$:Type, $l_{1}$, $l_{2}$:$T$ List, $x$:$T$. $l_{1}$ $\leq$ $l_{2}$ @ [$x$] $\Leftrightarrow$ $l_{1}$ $\leq$ $l_{2}$ $\vee$ $l_{1}$ $=$ ($l_{2}$ @ [$x$])