targetdoc attribute of olink

at http://www.sagehill.net/docbookxsl/OlinkResolving.html
Bob Stayton says the targetdoc attribte of olink is optional under
special circumstances.

The DocBook 5.0b7 schema does not appear to allow me to omit this
attribute, so I am wondering if there was a change in the intended
content model or if there was a small oversight. Thank you.

-- 
http://chris.chiasson.name/