XML Schemas - How would you specify attributes to be mutually exc lusive

[Anthony Dodd <An...@synergy-fs.com>]

Hi
   
  	I was just wondering if any body might be able to help me. I'm
attempting to build a schema to model a set of xml rules used for data
transformation. One of the thing I would like to do is 
to specify an element e.g. <X A="" | B=""/>, where element X must have one
of the attributes A or B but no both. With elements I could use the <choice>
grouping, is there some equivalent for attributes. Grateful for any help in
adavance.

Regards
Tony