title: Make new UINavigationController then empty stack of old navigation controller programatically
body: <p>Without using storyboards, I want to:</p>
  
  <ol>
  <li><p>Create a UINavigationController (let's call it NC_A) in the app delegate, then push a bunch of UIViewControllers onto it. (This doesn't need to be explained. It's just step one.)</p></li>
  <li><p>Then, from the last UIViewController on the stack, I want to jump off to a new UINavigationController (NC_B).</p></li>
  <li><p>After the new NC_B's root view is shown, behind the scenes on NC_A pop back to NC_A's root view so that when I return to it, the root view is shown.</p></li>
  </ol>