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

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

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

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