how to match non-mixed content?

How do I transform this ...

        <p><strong>Some Heading</strong></p>

.... to this ...

        <h2>Some Heading</h2>

.... but to deal with the following as any typical paragraph mixed 
content?
        <p>Some person said <strong>something</strong></p>

Or, put differently, what xpath expression do I use to get the xhtml:p 
elements that contain a xhtml:strong element, without mixed content?
Bruce