$\forall$$T$:Type, $L_{1}$, $L_{2}$:($T$ List). fseg($T$;$L_{1}$;$L_{2}$) $\Leftarrow\!\Rightarrow$ ($\exists$$n$:\{0..($\parallel$$L_{2}$$\parallel$+1)$^{-}$\}. ($L_{1}$ = nth\_tl($n$;$L_{2}$)))