'A |SettingsPart| object providing access to the document-level
settings for this document. Creates a default settings part if one is
not present.'

@property
def _settings_part(self):

    try:
        return self.part_related_by(RT.SETTINGS)
    except KeyError:
        settings_part = SettingsPart.default(self.package)
        self.relate_to(settings_part, RT.SETTINGS)
        return settings_part
