Use xsd to specify multiple instances of existing element (Plain text resubmission)

Sorry about the unnecessary formatting clutter.

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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