[xsl] How to extract an element plus all the elements it references plus

Hi Folks,

I have instance documents containing a bunch of <airport> elements. I want to 
extract one of the <airport> elements. But the <airport> element might contain 
IDREF elements, so the result document must include the <airport> element and 
the referenced elements. Those referenced elements might contain IDREF 
elements, so the result document must include those referenced elements as 
well. And on and on and on. Further, while collecting those referenced 
elements, there might be a reference to an element that has already been 
collected; I don't want to collect elements twice.

Here is a sample instance document:

<Airports>
    <airport id="A">
        <ICAO>KBOS</ICAO>
        <ref>A-1</ref>
    </airport>
    <airport id="B">
        <ICAO>KSTL</ICAO>
        <ref>B-1</ref>
    </airport>
    <runways id="A-1">
        <runway>
            <ref>A-1-1</ref>
        </runway>
        <runway>
            <ref>A-1-2</ref>
        </runway>
    </runways>
    <runways id="B-1">
        <runway>
            <ref>B-1-1</ref>
        </runway>
    </runways>
    <gate id="A-1-1">
        <ref>A</ref>
    </gate>
    <gate id="A-1-2" />
    <gate id="B-1-1" />
</Airports>

I would like to extract the <airport> element with ICAO = KBOS (Boston 
airport). The following result document contains the desired <airport> element 
plus all the recursively referenced elements:

<Result>
    <airport id="A">
        <ICAO>KBOS</ICAO>
        <ref>A-1</ref>
    </airport>
    <runways id="A-1">
        <runway>
            <ref>A-1-1</ref>
        </runway>
        <runway>
            <ref>A-1-2</ref>
        </runway>
    </runways>
    <gate id="A-1-1">
        <ref>A</ref>
    </gate>
    <gate id="A-1-2" />
</Result>

I've written an XSLT program to do the job. See below. I am wondering if there 
is a better (simpler, clearer) solution?

/Roger

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform";
    xmlns:f="function"
    exclude-result-prefixes="f"
    version="2.0">
    
    <xsl:output method="xml" />
    
    <xsl:variable name="doc" select="/"/>
    
    <xsl:template match="Airports">
        <Results>
            <xsl:sequence 
select="f:get-elements-and-their-referenced-elements(airport[ICAO eq 'KBOS'], 
foo)" />
        </Results>
    </xsl:template> 
    
    <xsl:function name="f:get-elements-and-their-referenced-elements" 
as="element()*">
        <xsl:param name="elements-to-process" as="element()*" />
        <xsl:param name="processed-elements" as="element()*" />
        
        <xsl:choose>
            <xsl:when test="not($elements-to-process)">
                <xsl:sequence select="$processed-elements" />
            </xsl:when>
            <xsl:otherwise>
                <xsl:variable name="referenced-elements" as="element()*">
                    <xsl:for-each select="$elements-to-process//ref">
                        <xsl:variable name="ref" select="." />
                        <xsl:sequence select="$doc//*[@id eq $ref]" />
                    </xsl:for-each>
                </xsl:variable> 
                <xsl:variable name="new-elements-to-process" 
select="$referenced-elements except $processed-elements" />
                <xsl:variable name="new-processed-elements" 
select="$processed-elements union $new-elements-to-process" />
                <xsl:sequence 
select="f:get-elements-and-their-referenced-elements($new-elements-to-process, 
$new-processed-elements)" />
            </xsl:otherwise>
        </xsl:choose>
        
    </xsl:function> 
    
</xsl:stylesheet>
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