xml schema keyref 跨文件外键关联

Question

stu.xsd
-----------------------------------------------------------------------------------------------------<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
 xmlns:cla="http://www.power.com/class" >
<xs:import namespace="http://www.power.com/class" schemaLocation="class.xsd"/>
<xs:element name="student">
<xs:complexType>
<xs:all>
<xs:element name="stu_id" type="xs:int"/>
<xs:element name="name" type="xs:string"/>
<xs:element name="class_id" type="xs:int"/>
</xs:all>
</xs:complexType>
<xs:key name="key_sut">
<xs:selector xpath=".//stu_id"></xs:selector>
<xs:field xpath="."></xs:field>
</xs:key>
<xs:keyref refer="cla:key_class" name="keyref_class">
<xs:selector xpath=".//class_id"></xs:selector>
<xs:field xpath="."></xs:field>
</xs:keyref>
</xs:element>
</xs:schema>
class.xsd
-----------------------------------------------------------------------------------------------------
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
 xmlns:cla="http://www.power.com/class" targetNamespace="http://www.power.com/class">
<xs:element name="class">
<xs:complexType>
<xs:all>
<xs:element name="class_id" type="xs:int"/>
<xs:element name="class_name" type="xs:string"/>
</xs:all>
</xs:complexType>
<xs:key name="key_class">
<xs:selector xpath=".//class_id"></xs:selector>
<xs:field xpath="."></xs:field>
</xs:key>
</xs:element>
</xs:schema>
错误区域：
-----------------------------------------------------------------------------------------------------<xs:keyref refer="cla:key_class" name="keyref_class">
    <xs:selector xpath=".//class_id"></xs:selector>
    <xs:field xpath="."></xs:field>
</xs:keyref>不知道如何是好，才能达到跨文件外键关联。。