请问一下在xml schema中，我想给firstname标签限制属性怎么限制

Question

xml文件<?xml version="1.0" encoding="iso-8859-1"?><employee xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:noNamespaceSchemaLocation="note.xsd">    <firstname></firstname></employee>xsd文件<?xml version="1.0"?><xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">   <xs:attributeGroup name="asd">    <xs:attribute name="siteId" type="xs:string" use="required"/>    <xs:attribute name="lastname" type="xs:string" use="required"/>  </xs:attributeGroup>  <xs:element name="employee">    <xs:complexType>      <xs:sequence>          <xs:element name="firstname" type="xs:string"/>      </xs:sequence>    </xs:complexType>  </xs:element>  <xs:element name="firstname">    <xs:complexType>      <xs:attributeGroup ref="asd"/>    </xs:complexType>  </xs:element></xs:schema>知道的请说一下，谢谢