Merge XML documents

<mapping>
    <key value="assigned">
        <a/>
    </key>
    <whatever attribute="x">
        <k/>
        <j/>
    </whatever>
</mapping>

<mapping>
    <key value="identity">
        <a/>
        <b/>
    </key>
</mapping>

<mapping>
    <key value="identity">
        <a/>
        <b/>
    </key>
    <whatever attribute="x">
        <k/>
        <j/>
    </whatever>
</mapping>