DOCBOOK: ulink in bibliomixed

Why is ulink not allowed in bibliomixed? I'd like to refer to the page
where a document is available for purchase, like this:

    <bibliomixed>
      <abbrev>ISO14482</abbrev> <title>ISO/IEC 14482-1998 &mdash; The
      C++ Programming Language</title>. Available as PDF and as
      printed book from http://webstore.ansi.org/.
    </bibliomixed>

Thank you.

-- 
Aaron Isotton

http://www.isotton.com/
My GPG Public Key: http://www.isotton.com/gpg-public-key