The helper provides options for arbitrary HTML to be included at the topandbottom of the list — you saw them in the earlier snippet from the layout body (Listing 2) as :topand :bottom.
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。
2
The top 62 saw their net worth rise by more half a trillion dollars between 2010 and 2015, while the 3.6 billion people in the bottom half of the heap lost a trillion dollars.