Design of Pier Blog

Hello, I am trying to figure out the design of the classes in Pier Blog. I am wondering why PBBlog is a subclass of PRCase, and not a direct subclass of PRStructure: PRStructure #('parent' 'name' 'title' 'tags') PRCase #('document') PBBlog #('managingEditor' 'webMaster' 'copyright' 'language' 'itemCount' 'feedTitle' 'commentTimeout') PBEntry #('uuid' 'author' 'publication') PBComment #() PBPost #('sourceUrl' 'sourceTitle' 'enclosure') PRPage #() PRComponent #('componentClass' 'settings') PRFile #('file') PRForm #('model' 'metamodel') PRCase's class comment says:"I am an abstract class holding onto a document definition representing the contents of the receiver. Most of my subclasses will allow user to edit myself using the Wiki syntax." I guess my question is: for an application that is similar to a blog (managing a list of homogeneous entries, maybe nested), is it a good idea to make it a direct subclass of PRStructure? Matthias