<xs:attributeGroup name="arcAttrs">
    <xs:attribute ref="xlink:type" fixed="arc" use="required" />
    <xs:attribute ref="xlink:arcrole" />
    <xs:attribute ref="xlink:title" />
    <xs:attribute ref="xlink:show" />
    <xs:attribute ref="xlink:actuate" />
    <xs:attribute ref="xlink:from" />
    <xs:attribute ref="xlink:to">
        <xs:annotation>
            <xs:documentation>
     from and to have default behavior when values are missing
    </xs:documentation>
        </xs:annotation>
    </xs:attribute>
</xs:attributeGroup>