Reg. <all> model group extension

Hi All,
 
I have one doubt reg. <all> model group extension.
 
i have the following schema. 
 
<schema targetNamespace="testschema" xmlns:tns="testschema">
    <complexType name="BaseType">
        <all>
            <element name="a1" type="string"/>
            <element name="a2" type="string"/>
        </all>
    </complexType>
 
    <complexType name="DerivedType">
        <complexContent>
          <extension base="tns:BaseType">
            <all>
                <element name="a3" type="string"/>
            </all>
        <extension>
        </complexContent>
    <complexType>
 
</schema>
 
Is this extension of the <all> model group correct?? 
If not please provide me the reason. Please reply me as soon as possible.
 
 
Thanks and Regards,
Bharath.