freePositions :: Board -> Set.Set Position
freePositions board = Set.filter (flip Map.notMember $ board&pieces)
                    . allPositions
                    $ board
freePositions :: Board -> Set.Set Position
freePositions board = Set.filter (flip Map.notMember $ board&pieces)
                    . allPositions
                    $ board
freePositions board = Set.filter (flip Map.notMember $ board&pieces)
                    . allPositions
                    $ board