xsl:sort for the first child element

Hello,

I have a problem regarding xsl:sort. Using an XML element like this.

<Bibitem Type="Book" Label="TEI">
        <Editor>
                <Given>C.M.</Given>
                <Family>Sperberg-McQueen</Family>
        </Editor>
        <Editor>
                <Given>L.</Given>
                <Family>Burnard</Family>
        </Editor>
        <Title>TEI P4: Guidelines for Electronic Text Encoding and 
Interchange</Title>
        <Address>XML Version: Oxford, Providence, Charlottesville, 
Bergen</Address>
        <Year>2002</Year>
</Bibitem>

it is no problem to sort the Bibitems according their Author or Editor 
elements with this stylesheet template:
...
<xsl:if test="Bibitem/@Type='Book'">
        <xsl:for-each select="Bibitem[(_at_)Type='Book']">
                <xsl:sort select="Author[1]/Family | Editor[1]/Family | 
Organization"/>
                <p>
                        <xsl:apply-templates select="."/>
                </p>
        </xsl:for-each>
</xsl:if>
...

Note that all three elements are allowed as 'author' of a Bibitem: Author, 
Editor and Organization.
The problem is that if I use a Bibitem with both an Author or Editor 
element and an Organization element as well (which is also allowed 
according to the DTD), the xsl:sort breaks ("a sequence of more than one 
item is not allowed here" - of course). XSLT processor is Saxon 7.5.1
<Bibitem Type="Book" Label="TEI">
        <Editor>
                <Given>C.M.</Given>
                <Family>Sperberg-McQueen</Family>
        </Editor>
        <Editor>
                <Given>L.</Given>
                <Family>Burnard</Family>
        </Editor>
        <Title>TEI P4: Guidelines for Electronic Text Encoding and 
Interchange</Title>
        <Organization>Text Encoding Initiative Consortium</Organization>
        <Address>XML Version: Oxford, Providence, Charlottesville, 
Bergen</Address>
        <Year>2002</Year>
</Bibitem>

I tried to use xsl:sort as child element of xsl:choose/xsl:when, but it's 
not allowed. What I need is to check that the xsl:sort is only used on the 
first child element of Bibitem - regardless if it is Author, Editor or 
Organization. I tried the following without any success
<xsl:sort select="child::Author[1]/Family[position()=1] | 
child::Editor[1]/Family[position()=1] | child::Organization[position()=1]"/>
I know there were a lot of questions about getting the first child element, 
but I found none regarding xsl:sort. I had a look at Dave Pawson's FAQ too.
Any help would be appreciated.

Kind regards,

Maik Stührenberg


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