title: How to convert reverse iterator into forward iterator when it is rend?
body: <p>The normal way to convert from reverse to forward iterator is</p>
  
  <p><code>std::prev(r_it.base())</code></p>
  
  <p>Is this valid when <code>r_it</code> is the same as <code>rend()</code>?
  If not, how to safely convert any reverse iterator to forward iterator, where result of <code>rend()</code> converts to result of <code>end()</code></p>