title: Why we need rbegin and rend?
body: <p>Since we now have <code>advance()</code> and the <code>prev()</code> to move iterator to go front or go back, and we already have <code>begin()</code> and <code>end()</code>. </p>
  
  <p>I wonder is there any situation we better/have to move reverse iterator back and front? </p>