Prefix -> Namespace Resolution

Is there a way to determine the namespace URI associated with a given
prefix while processing a document?  I realize that namespace-uri will
give me the URI if I have an element.  In my case, I have an attribute
value that is a Qname, and I need to determine what the URI is for that
value.  For example:

<doc xmlns:x="http://someuri.org/">
	<foo myAttribute="x:myVal"/>
</doc>

In the course of processing, I will discover the value of myAttribute
for element foo, and I need to determine the namespace URI associated
with prefix x.  I appreciate any help you can give me.

Thanks,
Chris Dix


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