xmllint.exe --xpath " //header[ contains(.,'rationale') ]/l /preceding-sibling::* " fudge
xmllint.exe --xpath " //header[ contains(.,'rationale') ]/child::text() " fudge

##</code><code>##

<fudge> 
    <root>
        <part>
            <sect>
               <header>
                   1. Purpose and rationale
                   <p>purpose 1</p>
                   <p>Purpose 2</p>
                   <p>purpose 3</p>
                   <l>
                      <li>purpose list 1</li>
                      <li>list2</li>
                    </l>                      
                </header>
            </sect>
        </part>
    </root>
    <root>
        <part>
            <sect>
                <header>
                    2. Purpose and rationale
                    <p>2 purpose 1</p>
                    <p>2 Purpose 2</p>
                    <p>2 purpose 3</p>
                    <l>
                        <li>2 purpose list 1</li>
                        <li>2 list2</li>
                    </l>                      
                    <p>2. Some other heading</p>
                    <p>2 content 1</p>
                    <p>2 content 2</p>
                </header>
            </sect>
        </part>
    </root>
</fudge>
##</code><code>##

xmllint.exe --xpath " //header[ contains(.,'rationale') ]/child::text() | //header[ contains(.,'rationale') ] /l/preceding-sibling::*   " fudge


                   1. Purpose and rationale
                   <p>purpose 1</p>
                   <p>Purpose 2</p>
                   <p>purpose 3</p>


                    2. Purpose and rationale
                    <p>2 purpose 1</p>
                    <p>2 Purpose 2</p>
                    <p>2 purpose 3</p>