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