如何写一个dtd文件

class.dtd

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

class.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE classrooms SYSTEM "class.dtd">
<classrooms>
    <classroom id="c1">
        <grade>2010</grade>
        <classname>10级计算机应用技术一班</classname>
        <students>
            <student>
                <id>1</id>
                <studentname>宋发准</studentname>
                <age>12</age>
            </student>
            <student>
                <id>1</id>
                <studentname>宋发准</studentname>
                <age>12</age>
            </student>
        </students>
    </classroom>
    <classroom id="c2">
        <grade>2010</grade>
        <classname>10级计算机应用技术二班</classname>
        <students>
            <student>
                <id>2</id>
                <studentname>李四</studentname>
                <age>22</age>
            </student>
            <student>
                <id>2</id>
                <studentname>李四</studentname>
                <age>22</age>
            </student>
        </students>
    </classroom>
</classrooms>

?=0次或者1次,+=一次或者多次,*=0次或者多次