$\forall$$T$:Type, $l_{1}$, $l_{2}$:($T$ List).
\\[0ex]fseg($T$;$l_{1}$;$l_{2}$)
\\[0ex]$\Leftarrow\!\Rightarrow$ (($\parallel$$l_{1}$$\parallel$ $\leq$ $\parallel$$l_{2}$$\parallel$) c$\wedge$ ($\forall$$i$:$\mathbb{N}$. ($i$ $<$ $\parallel$$l_{1}$$\parallel$) $\Rightarrow$ ($l_{1}$[$i$] = $l_{2}$[(($\parallel$$l_{2}$$\parallel$ {-} $\parallel$$l_{1}$$\parallel$)+$i$)] $\in$ $T$)))