Here we develop some thoughts on the capability for category systems that might be added to wiki - specifically, 'book-style' organisation of extended topics. Category: Method
> For categories specifically deployed in foprop see Categories in foprop.
Here we explore some requirements for pages organised (hierarchically?) under categories. Earlier, more general thinking about categories and tags, and reasons for needing them.
A hierarchy of book/library-style categories
Book category hierarchy
Required: A plugin that 'knows' the above category hierarchy, where the author can: - Tag the present page (book title, page title, chapter title etc). Only 'Page 1' pages (title pages) get tagged, subsidiary pages are tacit; inclusion is defined by links. - Identify the Book or Collection that it is part of - Allocate a tacit sequence number (for a part/chapter/section)
Book-style plugin display
Describes this page lineup 
The plugin will - Survey the Book-style categories in the present collection/book - Display a half-width paragraph which - Displays the page name, highlighted (header-size text? page-title size text?) - Perhaps with colour coding (a paragraph width horizontal bar) - In its correct position within the collection/book/part/chapter hierarchy. Clickable links. And - Automatically generate a 'Contents' ghost page for the Book category, with a clickable link.
Parhaps also: - Create a GraphViz map paragraph