docs: 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