[#16] "Article" represents a page or a post which consists of web-sites.
[#16] "List" represents a post-list which consists of web-sites.
[#16] "UrlJoin" generate a relative location from fragments of a URL.
[#16] An instance of "List" should have a method "Content".
[#16] An instance of "List" needs not have a list of "Article" instances.
[#16] "UrlJoin" requires variable-length arguments. Each of them is typed as string.
[#16] Convert values typed as int to string to generate the location from them.
[#16] Rename "_list.go" to "content_list.go" to build.