$\forall$$T$:Type, ${\it eq}$:EqDecider($T$), ${\it as}$, ${\it bs}$:$T$ List, $x$:$T$.
\\[0ex]($x$ $\in$ list{-}diff(${\it eq}$;${\it as}$;${\it bs}$)) $\Leftrightarrow$ ($x$ $\in$ ${\it as}$) \& $\neg$($x$ $\in$ ${\it bs}$)