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

def _next_rId(self):
        """
        Next available rId in collection, starting from 'rId1' and making use
        of any gaps in numbering, e.g. 'rId2' for rIds ['rId1', 'rId3'].
        """
        for n in range(1, len(self)+2):
            rId_candidate = 'rId%d' % n  # like 'rId19'
            if rId_candidate not in self:
                return rId_candidate