$\forall$$T$:Type, $L_{1}$,$L_{2}$:($T$ List).
\\[0ex]iseg($T$; $L_{1}$; $L_{2}$) $\Leftarrow\!\Rightarrow$ ($\exists$$n$:int\_seg(0; ($\parallel$$L_{2}$$\parallel$ + 1)). ($L_{1}$ = firstn($n$; $L_{2}$)))