<!DOCTYPE book SYSTEM "subdoc.dtd" [
<!ENTITY subdoc1 SYSTEM "subdoc1.sgm" NDATA SGML>
<!ENTITY subdoc2 SYSTEM "subdoc2.sgm" NDATA SGML>
]>
<book><title>Book Title</title>
<chapter><title>Chapter Title</title>
<para>
This is a test.
</para>
<subdocsection subdoc=subdoc1>
<subdocsection subdoc=subdoc2>
</chapter>
</book>