extend two elements?

Hi,

I have an element called "grad_student" which extends "student". Now I want to extend "grad_address" element too since "grad_student" element needs address related info. Unfortunately I cannot merge address info from "grad_address" element into "grad_student" element. I am wondering how I can
extend another element when this element already extends a base element?

regards,

    <xsd:complexType name="grad_student">
          <xsd:complexContent>
             <xsd:extension base="student">
                <xsd:attribute name="name" type="xsd:string" use="required"/>
                <xsd:attribute name="grade" type="xsd:string" use="required"/>
             </xsd:extension>
         
 </xsd:complexContent>
       </xsd:complexType>
       <xsd:element name="grad_student" type="xyz:grad_student"/>
       
          <xsd:complexType name="grad_address">
             <xsd:complexContent>
                <xsd:extension base="address"> 
                   <xsd:attribute name="state" type="xsd:string" use="required"/>
                   <xsd:attribute name="zip_code" type="xsd:string" use="required"/>
                </xsd:extension>
             </xsd:complexContent>
          </xsd:complexType>
       <xsd:element name="grad_address" type="xyz:grad_address"/>

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