XSD Prob

Hi all,
Is there a way to write a xsd which validates a xml which is having
something like this.
 
<a b="c">
<c1>test c1</c1>
<c2>test c2</c2>
</a>
 
that means depending on the value of the attribute 'b' the corresponding
elements differ.i.e if value is 'c' then some elements are displayed if
its 'd' then some elements.can we write XSD to support this kind of
requirement.Kindly help me.
thanks and regards
aaaaa

Confidentiality Notice 

The information contained in this electronic message and any attachments to this message are intended
for the exclusive use of the addressee(s) and may contain confidential or privileged information. If
you are not the intended recipient, please notify the sender at Wipro or Mailadmin@wipro.com immediately
and destroy all copies of this message and any attachments.