<xs:attribute name="type">
    <xs:simpleType>
        <xs:restriction base="xs:token">
            <xs:enumeration value="hasformat" />
            <xs:enumeration value="haspart" />
            <xs:enumeration value="hasversion" />
            <xs:enumeration value="isformatof" />
            <xs:enumeration value="ispartof" />
            <xs:enumeration value="isreferencedby" />
            <xs:enumeration value="isreplacedby" />
            <xs:enumeration value="isrequiredby" />
            <xs:enumeration value="isversionof" />
            <xs:enumeration value="references" />
            <xs:enumeration value="replaces" />
            <xs:enumeration value="requires" />
            <xs:enumeration value="othertype" />
        </xs:restriction>
    </xs:simpleType>
</xs:attribute>