<?xml version="1.0" encoding="UTF-8"?>
<!ELEMENT book (title, author+, isbn, subject*, publisher, date?)>
<!ELEMENT author (#PCDATA)>
<!ELEMENT isbn (#PCDATA)>
<!ELEMENT publisher (#PCDATA)>
<!ELEMENT subject (#PCDATA)>
<!ELEMENT title (#PCDATA)>
<!ELEMENT date (#PCDATA)>