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