title: Single activity Fragment based app and back button
body: <p>I got the app, that uses practcally just one activity. There is main area where I put fragments into. But what about back button now?</p>
  
  <p>Of course I can override <code>onBackPressed()</code> but with what?</p>
  
  <p>I'm pushing a fragment into it's holder using <code>FragmentTransaction.replace()</code> method every time. I might be lacking understanding of a subject, but shouldn't there be some fragment built-in stack, that would allow me to point <code>onBackPressed()</code> to the previous fragment in stack?</p>