<!ELEMENT book ( title | author | isbn | cat | review | extract )* >
<!ATTLIST book id CDATA #REQUIRED >

<!ELEMENT author ( #PCDATA )>

<!ELEMENT cat ( #PCDATA ) >

<!ELEMENT extract ( #PCDATA ) >

<!ELEMENT isbn ( #PCDATA ) >

<!ELEMENT review ( #PCDATA ) >

<!ELEMENT title ( #PCDATA ) >