<?xml version="1.0"?>
<!DOCTYPE book [
<!ELEMENT book (doc)*>
<!ELEMENT doc (isid)*>
<!ELEMENT isid EMPTY>
<!ATTLIST isid myid ID #IMPLIED>
]>
<book>
<doc>
<isid myid="mine"/>
<isid myid="dup"/>
</doc>
<!-- including another XML document with IDs -->
<doc xml:base="../ents/ids.xml">
<isid myid="dup"/>
<isid myid="foo"/>
<isid myid="bar"/>
</doc>
</book>