<xs:attribute name="function">
    <xs:simpleType>
        <xs:restriction base="xs:token">
            <xs:enumeration value="alt" />
            <xs:enumeration value="backspace" />
            <xs:enumeration value="command" />
            <xs:enumeration value="control" />
            <xs:enumeration value="delete" />
            <xs:enumeration value="down" />
            <xs:enumeration value="end" />
            <xs:enumeration value="enter" />
            <xs:enumeration value="escape" />
            <xs:enumeration value="home" />
            <xs:enumeration value="insert" />
            <xs:enumeration value="left" />
            <xs:enumeration value="meta" />
            <xs:enumeration value="option" />
            <xs:enumeration value="pagedown" />
            <xs:enumeration value="pageup" />
            <xs:enumeration value="right" />
            <xs:enumeration value="shift" />
            <xs:enumeration value="space" />
            <xs:enumeration value="tab" />
            <xs:enumeration value="up" />
            <xs:enumeration value="other" />
        </xs:restriction>
    </xs:simpleType>
</xs:attribute>