module DataBase   where

import qualified Data.Set

{-@ (\\) :: Eq a => forall<p :: a -> Prop>. xs:[a<p>] -> ys:[a] -> {v:[a<p>] | (listElts v)  = (Set_dif (listElts xs) (listElts ys))} @-}
(\\) :: Eq a => [a] -> [a] -> [a]
[]     \\ _ = []
(x:xs) \\ ys = if x `elem` ys then xs \\ ys else x:(xs \\ ys)