docs: def num_having_numId(self, numId):
          """
          Return the ``<w:num>`` child element having ``numId`` attribute
          matching *numId*.
          """
          xpath = './w:num[@w:numId="%d"]' % numId
          try:
              return self.xpath(xpath)[0]
          except IndexError:
              raise KeyError('no <w:num> element with numId %d' % numId)