XML schema cos-nonambig problem

This error message below is reported back from Xerces. The problem is  
the "major" element. How can I reconstruct my XML Schema so that I  
can avoid this error?

cos-nonambig: "http://www.polopoly.com/polopoly/cm/app/xml":major and  
"http://www.polopoly.com/polopoly/cm/app/xml":major (or elements from  
their substitution group) violate "Unique Particle Attribution".  
During validation against this schema, ambiguity would be created for  
those two particles.
		

      <xs:complexType name="contentiddef">
		<xs:choice>
			<xs:choice minOccurs="0" maxOccurs="unbounded">
				<xs:element ref="x:major"/>
				<xs:element ref="x:externalid"/>
			</xs:choice>
			<xs:choice minOccurs="0" maxOccurs="unbounded">
				<xs:element ref="x:major"/>
				<xs:element ref="x:minor"/>
				<xs:element ref="x:version"/>
			</xs:choice>
		</xs:choice>
	</xs:complexType>

Thanks!

Niels Peter