title: HEAD~ vs HEAD^ vs HEAD@{} also known as tilde vs caret vs at sign
body: <p>HEAD is a pointer at the current branch. I have seen a variety of notations for ancestors of HEAD including </p>
  
  <ul>
  <li><code>HEAD~2</code></li>
  <li><code>HEAD^2</code></li>
  <li><code>HEAD@{2}</code></li>
  <li><code>HEAD~~</code></li>
  <li><code>HEAD^^</code></li>
  </ul>
  
  <p>What does each of the above mean, exactly? Where is the documentation for this? </p>