Breaking a Document into Physical Chunks

<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V3.1//EN" [
<!ENTITY chap1 SYSTEM "chap1.sgm">
<!ENTITY chap2 SYSTEM "chap2.sgm">
<!ENTITY chap3 SYSTEM "chap3.sgm">
<!ENTITY appa SYSTEM "appa.sgm">
<!ENTITY appb SYSTEM "appb.sgm">
]>
<book><title>My First Book</title>
&chap1;
&chap2;
&chap3;
&appa;
&appb;
</book>