Conditional Enum

Hi, 
I have a simple xml element which is an enum as :
<xs:simpleType name="a_type">
<xs:restriction base="xs:string">
<xs:enumeration value="XXX"/>
<xs:enumeration value="YYY"/>
</xs:restriction>
</xs:simpleType>
I want to define another simple element of type string which can take any value if the above a_type is XXX and another enum if the above a_type is YYY.
How do I do this.

Thanks


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Be a better friend, newshound, and 
know-it-all with Yahoo! Mobile.  Try it now.  http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ