code: def _core_properties_part(self):
          
          try:
              return self.part_related_by(RT.CORE_PROPERTIES)
          except KeyError:
              core_properties_part = CorePropertiesPart.default(self)
              self.relate_to(core_properties_part, RT.CORE_PROPERTIES)
              return core_properties_part
docs: |CorePropertiesPart| object related to this package. Creates
          a default core properties part if one is not present (not common).