docs: def _next_numId(self):
          """
          The first ``numId`` unused by a ``<w:num>`` element, starting at
          1 and filling any gaps in numbering between existing ``<w:num>``
          elements.
          """
          numId_strs = self.xpath('./w:num/@w:numId')
          num_ids = [int(numId_str) for numId_str in numId_strs]
          for num in range(1, len(num_ids)+2):
              if num not in num_ids:
                  break
          return num