xquery - cts search to test if the element is not available

Question

Below is the XML structure where I want to get the entries for which element co:isbn is not available:-

<tr:trackingRecord xmlns:tr="https://www.mla.org/Schema/Tracking/tr"
    xmlns:co="https://www.mla.org/Schema/commonModule/co"
    xmlns:r="http://www.rsuitecms.com/rsuite/ns/metadata">
    <tr:journal>
        <tr:trackingDetails>
            <tr:entry>
                <co:trackingEntryID>2015323313</co:trackingEntryID>
                <co:publicationDate>2015</co:publicationDate>
                <co:volume>21</co:volume>
            </tr:entry>
            <tr:entry>
                <co:trackingEntryID>2015323314</co:trackingEntryID>
                <co:publicationDate>2015</co:publicationDate>
                <co:isbn>
                    <co:entry>NA</co:entry>
                    <co:value>1234567890128</co:value>
                </co:isbn>
            </tr:entry>
            <tr:entry>
                <co:trackingEntryID>2015323315</co:trackingEntryID>
                <co:publicationDate>2015</co:publicationDate>
                <co:volume>21</co:volume>
                <co:isbn></co:isbn>
            </tr:entry>
            <tr:entry>
                <co:trackingEntryID>2015323316</co:trackingEntryID>
                <co:publicationDate>2015</co:publicationDate>
                <co:volume>21</co:volume>
            </tr:entry>
        </tr:trackingDetails>
    </tr:journal>
</tr:trackingRecord>

Please suggest the cts:query for the same.