\begin{tabbing}
l\_member!($x$; $l$; $T$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$\exists$\=$i$:$\mathbb{N}$\+
\\[0ex](($i$ $<$ $\parallel$$l$$\parallel$)
\\[0ex]c$\wedge$ (($x$ = $l$[$i$] $\in$ $T$) $\wedge$ ($\forall$$j$:$\mathbb{N}$. ($j$ $<$ $\parallel$$l$$\parallel$) $\Rightarrow$ ($x$ = $l$[$j$] $\in$ $T$) $\Rightarrow$ ($j$ = $i$ $\in$ $\mathbb{N}$))))
\-
\end{tabbing}