$\forall$$T$:Type, $L_{1}$,$L_{2}$:($T$ List). ($\neg$($\uparrow$null($L_{2}$))) $\Rightarrow$ sublist($T$; $L_{1}$; tl($L_{2}$)) $\Rightarrow$ sublist($T$; $L_{1}$; $L_{2}$)