Current citation (biblioref) proposal

This is the content model (in Relax NG compact syntax) that Bruce,
Markus, and Peter are proposing for the biblioref element:

  # define new biblioref element
     biblioref = element biblioref {
       biblioref.attlist
     }
     biblioref.attlist &=
         attribute linkend { xsd:IDREF },
         attribute endterm { xsd:IDREF }?,
         attribute unit { xsd:token }?,
         attribute start {xsd:token }?,
         attribute end { xsd:token }?,
         attribute renderas {
           "full" | "author" | "year" | "author-year" | "title" | "nocite"
         }?