XSD limitations!

[momotaro]

am trying to do this

<xs:element name="ENTITY">
        <xs:complexType>
            <xs:sequence>
                <xs:element ref="UNIQUE" minOccurs="0" maxOccurs="unbounded"/>
                <xs:element ref="FIELD" maxOccurs="unbounded"/>
                <xs:element ref="ASSOCIATION" minOccurs="0" maxOccurs="unbounded"/>
                <xs:element ref="NAME" minOccurs="0" maxOccurs="1"/>
                <xs:element ref="CODE" minOccurs="0" maxOccurs="1"/>
                <xs:element ref="DESC" minOccurs="0" maxOccurs="1"/>
            </xs:sequence>
            <xs:attributeGroup ref="EntityAtt"/>
        </xs:complexType>
    </xs:element>
 

with UNIQUE this way:

<xs:element name="UNIQUE">
        <xs:complexType>
            <xs:sequence minOccurs="0" maxOccurs="unbounded">
                <xs:element ref="FILED">
                </xs:element>
            </xs:sequence>
        </xs:complexType>
    </xs:element>
 

and I know that with XSD some stuff above is impossible so if you have any worksaround I'll be glad !

thx in advance.