Query : Number of Attributes under an element & atleast 1 occurence of element/Attribute

[kosaraju.puneeth]

I am using Xml schema based validation for XML files.

* I have attributes of Required,optional and anyAttribute type. Now
can i validate my XML file in such a way that under a particular
element i find only 2 attributes present(including anyAttribute type)?

* <a>
<b/> <!--Optional element/attribute-->
<c/> <!--Optional element/attribute-->
<d/> <!--Optional element/attribute-->
</a>

now i need to validate that atleast one of them should be present under
"a" in my XML instance file