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Yagsbook is an XML format similar to Docbook. Initially is was going to be a few addons to Docbook, but it was soon decided that Docbook was too tied to its own domain (technical documentation) to be useful for a roleplaying game. There are some similarities between Docbook and Yagsbook, but these are for historical reasons.
Documentation for Yagsbook is currently limited, but there are two sources which can be used.
There are three types of top level documents in Yagsbook and many more support documents which may be included by the top three.
The principle type of document in Yagsbook is the <article>. Articles are designed to be the top level holder of all other document types, and as such is the only document type which contains extensive header information.
Though bestiary and character documents are also considered top level, these can be referenced from an article and inlined. No other document type can currently inline an article.