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

def _settings_part(self):
        """
        A |SettingsPart| object providing access to the document-level
        settings for this document. Creates a default settings part if one is
        not present.
        """
        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