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

@lazyproperty
def numbering_part(self):

    try:
        return self.part_related_by(RT.NUMBERING)
    except KeyError:
        numbering_part = NumberingPart.new()
        self.relate_to(numbering_part, RT.NUMBERING)
        return numbering_part
