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

def num_having_numId(self, 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))
