Return the ``<w:num>`` child element having ``numId`` attribute
        matching *numId*.

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)