All horizontal squares from the piece's point of view.

        Returns a list of relative movements up to the board's bound.

def horizontals(self):
        """ All horizontal squares from the piece's point of view.

        Returns a list of relative movements up to the board's bound.
        """
        horizontal_shifts = set(izip_longest(map(
            lambda i: i - self.x, range(self.board.length)), [], fillvalue=0))
        horizontal_shifts.discard((0, 0))
        return horizontal_shifts