'The first ``numId`` unused by a ``<w:num>`` element, starting at
1 and filling any gaps in numbering between existing ``<w:num>``
elements.'

@property
def _next_numId(self):

    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
