<!ELEMENT catalog (book*)>
<!ELEMENT book (title, isbn, author_last_name, author_first_name,
                publisher, publish_date, category, price, review*)>
<!ELEMENT title (#PCDATA)>
<!ELEMENT isbn (#PCDATA)>
<!ELEMENT author_last_name (#PCDATA)>
<!ELEMENT author_first_name (#PCDATA)>
<!ELEMENT publisher (#PCDATA)>
<!ELEMENT publish_date (#PCDATA)>
<!ELEMENT category (#PCDATA)>
<!ELEMENT price (#PCDATA)>
<!ELEMENT review (#PCDATA)>