|CorePropertiesPart| object related to this package. Creates
        a default core properties part if one is not present (not common).

def _core_properties_part(self):
        """
        |CorePropertiesPart| object related to this package. Creates
        a default core properties part if one is not present (not common).
        """
        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