>  you can run it to see the result. 

yes but it's not clear what you wanted the result to be.

The result of your code as posted is:

<?xml version="1.0" encoding="UTF-8"?><p>barfoo</p>
    <p> foobar </p>



If I modify it as Andrew suggested, using the third argument of key to
tell the system to look up keys in the original document 

by (adding

<xsl:variable name="main" select="/"/>

as a global variable and changing
                    select="key('extra','introduction')">
to
                    select="key('extra','introduction',$main)">

I get

<?xml version="1.0" encoding="UTF-8"?><p>barfoo</p><p>barfoo</p>
    <p> foobar </p>

ie an extra copy of <p>barfoo</p> but I can't tell if that's the result
you want.

David

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