The only thing I did was inserting Bob's proposed extension line as the first line in the local part of the DTD:
xmllint --version using libxml version 20430
xmllint --valid --noout --catalogs skeleton.xml
skeleton.xml:25: validity error: NOTATION attribute application reference an unknown notation "PDF"
<book>
^
Xerces' DOMCount agrees:
Error at skeleton.xml.873543, line 21, char 4 Message: Attribute 'application' refers to an unknown notation 'PDF'
Tobi
-- http://www.pinkjuice.com/
