<?xml version="1.0" ?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.0//EN"
               "../dtd/4.0/docbookx.dtd" [
  <!ENTITY subdoc SYSTEM "subdoc.ent">
]>
<book lang="en">
<title>Main document</title>
<chapter id="main">
<para>This references to: <link linkend="subdoc"/></para>
</chapter>
&subdoc;
</book>