$\forall$$T$:Type, $L_{1}$,$L_{2}$,$L_{3}$:($T$ List). iseg($T$; $L_{1}$; $L_{2}$) $\Rightarrow$ compat($T$; $L_{2}$; $L_{3}$) $\Rightarrow$ compat($T$; $L_{1}$; $L_{3}$)