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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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