Need Help With XPath Predicate Issue

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XML

Hi all;

I've got an XPath expression with a predicate that selects
a person's name and ID from within a nested element, using
the XPathNavigator.Select method.

This works:
root/all_clients/client/client_name_and_ID[client_ID = 'xxx']

This also works:
root/all_clients/client[client_name_and_ID/client_ID = 'xxx']/client_name_and_ID

When I take the predicate to the next level, it does not work:
root/all_clients[client/client_name_and_ID/client_ID = 'xxx']/client/client_name_and_ID

I do not get any filtering, but the entire set.

Is this due to a limitation inherent within XPath, within ASP.NET,
or am I doing something stupid?

What follows is a snippet from the relevant XML file:

<<pre lang="xml">?xml version="1.0" encoding="utf-8"?>
<root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:noNamespaceSchemaLocation="file:///n:\Projects\XML\Medical\Example_01.xsd">
    <all_clients>
        <client>
            <client_name_and_ID>
                <first_name>Fred</first_name>
                <middle_name>James</middle_name>
                <last_name>Bowman</last_name>
                <client_ID>1</client_ID>
            </client_name_and_ID>
        </client>
        <client>
            <client_name_and_ID>
                <first_name>Mark</first_name>
                <middle_name>David</middle_name>
                <last_name>Colder</last_name>
                <client_ID>2</client_ID>
            </client_name_and_ID>
        </client>
        <client>
            <client_name_and_ID>
                <first_name>Joe</first_name>
                <last_name>Lewis</last_name>
                <client_ID>3</client_ID>
            </client_name_and_ID>
        </client>
        <client>
            <client_name_and_ID>
                <first_name>Sam</first_name>
                <last_name>Plank</last_name>
                <client_ID>4</client_ID>
            </client_name_and_ID>
        </client>
    </all_clients>
</root>

THANKS!!!