code: public void resetWithRoot(Activity activity, final Screen root) {
      checkOnCreateNotYetCalled(activity, "resetWithRoot() must be called before onCreate()");
      backStack.clear();
      backStack.push(root);
    }
docs: Clears the back stack of this Navigator and adds the input Screen as the new root of the back stack.
  
  @param activity  activity used to verify this Navigator is in an acceptable state when resetWithRoot is called
  @param root  new root screen for this Navigator
  @throws IllegalStateException if {@link #onCreate(Activity, Bundle)} has already been called on this Navigator