\begin{tabbing}
$\vdash$  \=$\forall$$T$:Type, $a$, $b$:($T$ List).\+
\\[0ex]($\parallel$$a$$\parallel$ = $\parallel$$b$$\parallel$) $\Rightarrow$ ($\forall$$i$:$\mathbb{N}$. ($i$ $<$ $\parallel$$a$$\parallel$) $\Rightarrow$ ($a$[$i$] = $b$[$i$])) $\Rightarrow$ ($a$ = $b$)
\-
\end{tabbing}