Use xsd to specify multiple instances of existing element

Hi all,
I'm trying to extend an existing schema.  The existing schema provides a Category element that takes a 'term' and a 'scheme' (yes, it's an xsd attempt at an Atom schema).  In my schema extension I'd like to specify that there will be two Category elements, one using a scheme of 'http://www.dig.com/privacy' that takes a term attribute of 'open' or 'private', and the second category element will take a scheme of 'http://www.dig.com/fishingType', taking a term attribute of either 'coarse' or 'open'.

Now, is it possible for an xsd schema to specify that these two elements must be present?  I can see how to mandate the presence of two different elements, but these represent the same element but with different attribute combinations.

cheers,
Ian


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Ian Mayo
PlanetMayo Ltd