writing schema for multiple prefix multiple namespaces

[PA]

Can anyone tell me how to write XDR schema for XML below:

<?xml version="1.0"?>
<bk:Book xmlns:bk="urn:BookSchema.xdr">
<a:title>Sam's Book</a:title>
<a:author>Sam</a:author>
<b:year>2003</b:year>
</bk:Book>

Thanks in advance.

Pratibha