title: Rolling back in Git
body: <p>I have committed, and pushed, several patches:
  A1-->A2-->A3-->A4 (HEAD)</p>
  
  <p>Everyone's pulled these changesets into their local copy.</p>
  
  <p>Now we want to "roll back" to A2, and continue developing from there - essentially throwing away A3 and A4. What's the best way to do this?</p>