$\forall$$T$:Type, $a$,$b$:$T$, $l_{1}$,$l_{2}$:($T$ List).
\\[0ex]iseg($T$; cons($a$; $l_{1}$); cons($b$; $l_{2}$)) $\Leftarrow\!\Rightarrow$ (($a$ = $b$) $\wedge$ iseg($T$; $l_{1}$; $l_{2}$))