Content model for the <conference> element, defined as an OR group of all members of the %conference.class; and the %x.class;
<!ENTITY % conference-model "(%conference.class; | %x.class;)*" >
(conf-date | conf-name | conf-num | conf-loc | conf-sponsor | conf-theme | conf-acronym | string-conf | x)*
archivecustom-models3.ent