-----------------</span>
<span class="comment">// -- adding   [6,9) -----------------------------------------</span>
<span class="comment">// in interval [4,6) intervals do not overlap</span>
<span class="comment">// in interval [6,8): 2 intervals overlap</span>
<span class="comment">// in interval [8,9) intervals do not overlap</span>
<span class="comment">// -----------------------------------------------------------</span>
<span class="comment">// -- adding   [1,9) ---------------------------------------