<xs:attribute name="class">
    <xs:simpleType>
        <xs:restriction base="xs:token">
            <xs:enumeration value="doi" />
            <xs:enumeration value="isbn" />
            <xs:enumeration value="isrn" />
            <xs:enumeration value="issn" />
            <xs:enumeration value="libraryofcongress" />
            <xs:enumeration value="pubsnumber" />
            <xs:enumeration value="uri" />
            <xs:enumeration value="other" />
        </xs:restriction>
    </xs:simpleType>
</xs:attribute>