-- |
-- Filter 'Position's only leaving those which have a 'Piece' in some board.
arePieces :: Board -> Set.Set Position -> Set.Set Position
arePieces board = Set.intersection (Map.keysSet (board&pieces)) . areValid board
arePieces :: Board -> Set.Set Position -> Set.Set Position
arePieces board = Set.intersection (Map.keysSet (board&pieces)) . areValid board
arePieces board = Set.intersection (Map.keysSet (board&pieces)) . areValid board