helper 函数允许列表的顶部和底部包含任意的 HTML —— 它们在前面的布局正文片段（清单 2）中是 :top 和 :bottom。 生成的 HTML 类似于清单 7。
The helper provides options for arbitrary HTML to be included at the top and bottom of the list — you saw them in the earlier snippet from the layout body (Listing 2) as :top and :bottom.
清单2 . iPhone主导航栏的布局正文。
这意味着必须为应用程序设置一个默认路径。这个应用程序没有呈现自己的文本，仅显示正文导航的布局。
This means you need to set up a default route for the application that renders no text of its own and just displays the layout with the main navigation.
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