<!ENTITY defsystem "<application>defsystem</application>">
<!ENTITY bookinfo SYSTEM "bookinfo.sgml">
<!ENTITY intro SYSTEM "intro.sgml">
+<!ENTITY notes SYSTEM "notes.sgml">
<!ENTITY ref SYSTEM "ref.sgml">
]>
<book>
&bookinfo;
&intro;
+¬es;
&ref;
</book>