<?xml version="1.0" encoding="UTF-8"?><!ELEMENT classromm (className,grade,students+)><!ATTLIST classromm id ID #REQUIRED><!ELEMENT className (#PCDATA)><!ELEMENT grade (#PCDATA)><!ELEMENT students (student+)><!ELEMENT student (id,stuName,age)><!ELEMENT id (#PCDATA)><!ELEMENT stuName (#PCDATA)><!ELEMENT age (#PCDATA)>

<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE classrom SYSTEM "classroom.dtd"><classrom id="c1">    <className>14bling</className>    <grade>2014</grade>    <students>        <student>            <id>1</id>            <stuName>bling</stuName>            <age>23</age>        </student>        <student>            <id>2</id>            <stuName>jun</stuName>            <age>20</age>        </student>    </students></classrom>