Required Titles

title.opt = title? & titleabbrev? & subtitle?
title.req = title  & titleabbrev? & subtitle?

info.notitle  =
   element info { (author|...)* }
info.titlereq =
   element info { title.req, (author|...)* }

element article { 
   (title.req, info.notitle) | info.titlereq,
   ...
}

(This isn’t exactly the same semantic.)