docs: def records0(self, tag=None):
          """Iterator over all level=0 records.
  
          :param str tag: If ``None`` is given (default) then return all level=0
              records, otherwise return level=0 records with the given tag.
          """
          _log.debug("in records0")
          for offset, xtag in self.index0:
              _log.debug("    records0: offset: %s; xtag: %s", offset, xtag)
              if tag is None or tag == xtag:
                  yield self.read_record(offset)