def shift(self, x=0, y=0):
        """Get the cell which is offset from this cell by x columns, y rows"""
        if not isinstance(x, int):
            assert y == 0, \
                "_XYCell.shift: x=%r not integer and y=%r specified" % (x, y)
            return self.shift(x[0], x[1])
        return self.table.get_at(self.x + x, self.y + y)._cell