$\forall$$T$:Type, $a$:$T$, ${\it as}$:$T$ List, $i$:$\mathbb{Z}$. 0$<$$i$ $\Rightarrow$ $i$$\leq\parallel$${\it as}$$\parallel$ $\Rightarrow$ ($a$.${\it as}$)$\backslash$[$i$] $=$ ($a$.${\it as}$$\backslash$[$i$$-$1])