I have two documents that I need to merge, that happen in a way that I don't seem to be able to find covered in other examples. Namely, that it needs to match not only on a node's attribute at one level, but also on the value of an attribute a node level below that, to get that node's value.

I'm trying to take this sample:

<?xml version="1.0" encoding="UTF-8" ?>
<marc:collection xmlns:marc="http://www.loc.gov/MARC21/slim"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <marc:record>
    <marc:datafield tag="035" ind1=" " ind2=" ">
        <marc:subfield code="a">12345</marc:subfield>
    </marc:datafield>
    <marc:datafield tag="041" ind1=" " ind2=" ">
        <marc:subfield code="a">eng</marc:subfield>
    </marc:datafield>
    <marc:datafield tag="650" ind1=" " ind2="4">
        <marc:subfield code="a">Art</marc:subfield>
    </marc:datafield>
    <marc:datafield tag="949" ind1=" " ind2=" ">
        <marc:subfield code="i">Review of conference proceedings</marc:subfield>
    </marc:datafield>
  </marc:record>
  <marc:record>
    <marc:datafield tag="035" ind1=" " ind2=" ">
        <marc:subfield code="a">54321</marc:subfield>
    </marc:datafield>
    <marc:datafield tag="041" ind1=" " ind2=" ">
        <marc:subfield code="a">eng</marc:subfield>
    </marc:datafield>
    <marc:datafield tag="650" ind1=" " ind2="4">
        <marc:subfield code="a">Byzantine</marc:subfield>
    </marc:datafield>
  </marc:record>
</marc:collection>

And when the value of "datafield" '035', "subfield" 'a' matches e.g. "12345"

<marc:collection xmlns:marc="http://www.loc.gov/MARC21/slim"
xmlns:fn="http://www.w3.org/2005/xpath-functions" xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:fo="http://www.w3.org/1999/XSL/Format">
  <marc:record>
    <marc:datafield ind2=" " ind1=" " tag="035">
        <marc:subfield code="a">12345</marc:subfield>
    </marc:datafield>
    <marc:datafield ind2="4" ind1=" " tag="650">
        <marc:subfield code="a">General works</marc:subfield>
        <marc:subfield code="x">Historians and critics</marc:subfield>
        <marc:subfield code="x">Smith, John, 1834-1917</marc:subfield>
    </marc:datafield>
    <marc:datafield ind2="4" ind1=" " tag="650">
        <marc:subfield code="a">Généralités</marc:subfield>
        <marc:subfield code="x">Historiens et critiques d'art</marc:subfield>
        <marc:subfield code="x">Dietrichson, Lorentz, 1834-1917</marc:subfield>
    </marc:datafield>
    <marc:datafield ind2=" " ind1=" " tag="654">
        <marc:subfield code="a">General works</marc:subfield>
    </marc:datafield>
    <marc:datafield ind2=" " ind1=" " tag="654">
        <marc:subfield code="a">Généralités</marc:subfield>
        <marc:subfield code="b">Historiens et critiques d'art</marc:subfield>
        <marc:subfield code="b">Smith, John, 1834-1917</marc:subfield>
    </marc:datafield>
  </marc:record>      
  <marc:record>
    <marc:datafield ind2=" " ind1=" " tag="035">
        <marc:subfield code="a">54321</marc:subfield>
    </marc:datafield>
    <marc:datafield ind2="4" ind1=" " tag="650">
        <marc:subfield code="a">General works</marc:subfield>
        <marc:subfield code="x">Historians and critics</marc:subfield>
        <marc:subfield code="x">Lange, Julius Henrik, 1838-1896</marc:subfield>
    </marc:datafield>
  </marc:record>
</marc:collection>

The result should be:

<?xml version="1.0" encoding="UTF-8" ?>
<marc:collection xmlns:marc="http://www.loc.gov/MARC21/slim"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <marc:record>
    <marc:datafield tag="035" ind1=" " ind2=" ">
        <marc:subfield code="a">12345</marc:subfield>
    </marc:datafield>
    <marc:datafield tag="041" ind1=" " ind2=" ">
        <marc:subfield code="a">eng</marc:subfield>
    </marc:datafield>
    <marc:datafield tag="650" ind1=" " ind2="4">
        <marc:subfield code="a">Art</marc:subfield>
    </marc:datafield>
    <marc:datafield ind2="4" ind1=" " tag="650">
        <marc:subfield code="a">General works</marc:subfield>
        <marc:subfield code="x">Historians and critics</marc:subfield>
        <marc:subfield code="x">Smith, John, 1834-1917</marc:subfield>
    </marc:datafield>
    <marc:datafield ind2="4" ind1=" " tag="650">
        <marc:subfield code="a">Généralités</marc:subfield>
        <marc:subfield code="x">Historiens et critiques d'art</marc:subfield>
        <marc:subfield code="x">Dietrichson, Lorentz, 1834-1917</marc:subfield>
    </marc:datafield>
    <marc:datafield ind2=" " ind1=" " tag="654">
        <marc:subfield code="a">General works</marc:subfield>
    </marc:datafield>
    <marc:datafield ind2=" " ind1=" " tag="654">
        <marc:subfield code="a">Généralités</marc:subfield>
        <marc:subfield code="b">Historiens et critiques d'art</marc:subfield>
        <marc:subfield code="b">Smith, John, 1834-1917</marc:subfield>
    </marc:datafield>
    <marc:datafield tag="949" ind1=" " ind2=" ">
        <marc:subfield code="i">Review of conference proceedings</marc:subfield>
    </marc:datafield>
  </marc:record>
  <marc:record>
    <marc:datafield tag="035" ind1=" " ind2=" ">
        <marc:subfield code="a">54321</marc:subfield>
    </marc:datafield>
    <marc:datafield tag="041" ind1=" " ind2=" ">
        <marc:subfield code="a">eng</marc:subfield>
    </marc:datafield>
    <marc:datafield tag="650" ind1=" " ind2="4">
        <marc:subfield code="a">Byzantine</marc:subfield>
    </marc:datafield>
    <marc:datafield ind2="4" ind1=" " tag="650">
        <marc:subfield code="a">General works</marc:subfield>
        <marc:subfield code="x">Historians and critics</marc:subfield>
        <marc:subfield code="x">Lange, Julius Henrik, 1838-1896</marc:subfield>
    </marc:datafield>
  </marc:record>
</marc:collection>

I've tried using examples that I've found that did lookups, but none of them seemed to work. I didn't include any of my XSL, because all of my results were disasterous. I keep looking at it, like it must be simple, but I'm just not getting any decent results. Any help or pointers would be greatly appreciated.

Thanks!