<xs:schema xmlns:xs=“www.w3.org/2001/XMLSchema” xmlns=“FOO” targetNamespace=“FOO”>

<xs:element name="foo">
        <xs:complexType>
                <xs:complexContent>
                        <xs:restriction base="typeA">
                                <xs:attribute name="barA" type="xs:string" use="optional"/>
                        </xs:restriction>
                </xs:complexContent>
        </xs:complexType>
</xs:element>

<xs:complexType name="typeA">
        <xs:attribute name="barA" type="xs:string" use="required"/>
</xs:complexType>

</xs:schema>