A |NumberingPart| object providing access to the numbering
        definitions for this document. Creates an empty numbering part if one
        is not present.

def numbering_part(self):
        """
        A |NumberingPart| object providing access to the numbering
        definitions for this document. Creates an empty numbering part if one
        is not present.
        """
        try:
            return self.part_related_by(RT.NUMBERING)
        except KeyError:
            numbering_part = NumberingPart.new()
            self.relate_to(numbering_part, RT.NUMBERING)
            return numbering_part