老师，为啥他这里要我强制转换成Attribute类型，我的course设置了cno属性啊

<?xml version="1.0" encoding="UTF-8"?>

<teaching-plan

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xsi:noNamespaceSchemaLocation="plan.xsd">

<course cno="s001">

<course-name>大学英语</course-name>

<class-hour>36</class-hour>

<exam-form>考试</exam-form>

</course>

<course cno="s002">

<course-name>高等数学</course-name>

<class-hour>70</class-hour>

<exam-form>考试</exam-form>

</course>

<course cno="s003">

<course-name>计算机应用基础</course-name>

<class-hour>108</class-hour>

<exam-form>上机考试</exam-form>

</course>

<course cno="s004">

<course-name>Java编程</course-name>

<class-hour>80</class-hour>

<exam-form>上机考试</exam-form>

</course>

</teaching-plan>

<?xml version="1.0" encoding="UTF-8"?>

<schema xmlns="http://www.w3.org/2001/XMLSchema">

<element name="teaching-plan">

<complexType>

<sequence>

<element name="course" minOccurs="1" maxOccurs="100">

<complexType>

<sequence>

<element name="course-name" type="string"></element>

<element name="class-hour">

<simpleType>

<restriction base="integer">

<minInclusive value="20"></minInclusive>

<maxInclusive value="110"></maxInclusive>

</restriction>

</simpleType>

</element>

<element name="exam-form" type="string"></element>

</sequence>

<attribute name="cno" type="string" use="required"></attribute>

</complexType>

</element>

</sequence>

</complexType>

</element>

</schema>