dynamic enumeration definition

Hi All,
 
I have a very strange problem regarding enumerations. I have 2 fields which have enumeration values associated with them. But the problem is based on first field's value, the second fields enumeration changes,
 
for example let us say I have 2 fields like below with their overall enumerations
 
field1 - road - {Pedestrain, Cycle Way, Bus-Taxi}
field2 - carriage - {Single Carriage, Dual Carriage, Single track}
 
if field1 is associated with a value of Cycle field2 can have only {Single Track and Dual Carriage}
but if field2 is associated with Bus-Taxi, field can have all of the valid values.
 
How do I represent such filtered enumerations using XML Schema?
 
Any help would be appreciated.
 
Thanks,
Shashi

		
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