type family Elem (x :: k) (xs :: [k]) :: Bool where
    Elem x '[] = 'False
    Elem x (x ': xs) = 'True
    Elem x (y ': xs) = Elem x xs

type family If (condition :: Bool) (t :: k) (e :: k) :: k where
    If 'True t e = t
    If 'False t e = e

type family Without (xs :: [k]) (ys :: [k]) :: [k] where
    Without xs '[] = xs
    Without '[] ys = '[]
    Without (x ': xs) ys = If (x `Elem` ys) (Without xs ys) (x ': Without xs ys)