'Next available rId in collection, starting from \'rId1\' and making use
of any gaps in numbering, e.g. \'rId2\' for rIds [\'rId1\', \'rId3\'].'

@property
def _next_rId(self):

    for n in range(1, (len(self) + 2)):
        rId_candidate = (u'rId%d' % n)
        if (rId_candidate not in self):
            return rId_candidate
