ing annotation id for this
            self._key_start = _KeyStartEndAnnid()
            self._by_start = SortedKeyList(key=self._key_start)
        else:
            # we sort by increasing start offset then increasing annotation id for this
            self._key_start = _KeyStartAnnid()
            self._by_start = SortedKeyList(key=self._key_start)
        # for this we sort by end offset only
        self._key_end = _KeyEndAnnid()
        self._by_end = SortedKeyList(key=self._key_end)

    def add(self