Safe but unreadable

gistfile1.txt

func pageViewController(pageViewController: UIPageViewController,
                        viewControllerBeforeViewController viewController: UIViewController) -> UIViewController? {
  guard let viewControllerIndex = orderedViewControllers.indexOf(viewController) where viewControllerIndex > 0 else {
    return nil
  }
  return orderedViewControllers[viewControllerIndex - 1]
}