       Args:
            start: the start offset of the span
            end: the end offset of the span, the offset of the first character after the span that does not
                belong to the span
        &#34;&#34;&#34;
        assert start is not None
        assert end is not None
        assert start &lt;= end
        self._start = start
        self._end = end

    @property
    def start(self):
        &#34;&#34;&#34;
        Return the start offset.
        &#34;&#34;&#34;
        return self.