docs: 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