def to_xml(self, xml_parent):
    axis = SubElement(xml_parent, 'axis')
    axis.set('xyz', '{0} {1} {2}'.format(self.x, self.y, self.z))
    return axis
def to_xml(self, xml_parent):
    axis = SubElement(xml_parent, 'axis')
    axis.set('xyz', '{0} {1} {2}'.format(self.x, self.y, self.z))
    return axis