Since preprocessing happens before htmlization but after a page is loaded and linkified, it should be possible to use it to create something like a link map or lists, or a page index. Page inlining and rss generation is already done via preprocessor directives and seems a natureal as a plugin too.
Note that things like a link map or a broken link list page would need to be updated whenever a set (or all) pages change; the %inlinepages hash already allows for pages to register this, although it might need to be renamed.
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+I need to look at the range of things that other wikis use their plugin systems for, but preprocessor directives as plugins certianly seems useful, even if it's not a complete solution.
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