<?xml version="1.0" encoding="UTF-8"?>
<!ELEMENT authors (author*)>
<!ELEMENT author (a_name,sex,email*)>
<!ATTLIST author id ID #REQUIRED>
<!ATTLIST author ISBN IDREFS #REQUIRED>
<!ELEMENT a_name (#PCDATA)>
<!ELEMENT sex (#PCDATA)>
<!ELEMENT email (#PCDATA)>
