$\forall$$T$:Type, ${\it as}$,${\it bs}$,${\it cs}$:($T$ List). iseg($T$; append(${\it as}$; ${\it bs}$); append(${\it as}$; ${\it cs}$)) $\Leftarrow\!\Rightarrow$ iseg($T$; ${\it bs}$; ${\it cs}$)