 all intervals which start at the last known start offset.
        &#34;&#34;&#34;
        laststart = None
        for intvl in reversed(self._by_start):
            if laststart is None:
                laststart = intvl[0]
                yield intvl
            elif intvl[0] == laststart:
                yield intvl
            else:
                return

    def min_start(self):
        &#34;&#34;&#34;
        Returns the smallest known start offset.
        &#34;&#34;&#34;
        return self._by_st