<xs:attributeGroup name="arcAttrs">
        <xs:attribute ref="xlink:type" use="required" fixed="arc"/>
        <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>
    <xs:group name="arcModel">
        <xs:sequence>
            <xs:element ref="xlink:title" minOccurs="0" maxOccurs="unbounded"/>
        </xs:sequence>
    </xs:group>
    <xs:complexType name="arcType">
        <xs:group ref="xlink:arcModel"/>
        <xs:attributeGroup ref="xlink:arcAttrs"/>
    </xs:complexType>