The content model for the <event> element.
<!ENTITY % event-model "(event-desc?, book-id*, edition*,
pub-date*, (%date.class;)*,
issn*, issn-l?, isbn*, permissions?,
notes*, self-uri*)" >(event-desc?, book-id*, edition*, pub-date*, (date | string-date)*, issn*, issn-l?, isbn*, permissions?, notes*, self-uri*)