title: Adding a Fragment with add() method doesn't hide previous fragments
body: <p>I'm experiencing "unexpected behaviour" when using add() method to add a new fragment.</p>
  
  <p>I want to add a new fragment on a FrameLayout, but when I do it the previous fragment stills visible.</p>
  
  <ul>
  <li><p>Is this the expected result when using add() method?</p></li>
  <li><p>It is because I am using a FrameLayout and add() method just place a fragment over the FrameLayout without affect the previous one?</p></li>
  </ul>
  
  <p>Thx</p>