'Parse the MusicXML <backup> element.
This moves the global time position backwards.
Args:
xml_backup: XML element with tag type \'backup\'.'

def _parse_backup(self, xml_backup):

    xml_duration = xml_backup.find('duration')
    backup_duration = int(xml_duration.text)
    midi_ticks = (backup_duration * (constants.STANDARD_PPQ / self.state.divisions))
    seconds = ((midi_ticks / constants.STANDARD_PPQ) * self.state.seconds_per_quarter)
    self.state.time_position -= seconds
