[xsl] Confusion about preceding-sibling axis

I'm a little confused about the preceding-sibling axis (or at least my
experience of it). Hopefully someone can point out my mistakes...

My original problem that required the use of the preceding-sibling axis was
to select a unique list of elements (using the familiar
*[not(@name=preceding-sibling::*/@name)] XPath pattern). I got unexpected
results so I simplified the XML and XSLT to investigate what was happening
(particularly with respect to using the preceding-sibling axis).

Consider the following simplified source XML:

<?xml version="1.0"?>
<root>
  <level1 id="1">
    <level2>
      <item>first item</item>
      <item>second item</item>
      <item>third item</item>
    </level2>
  </level1>
  <level1 id="2">
    <level2>
      <item>fourth item</item>
      <item>fifth item</item>
      <item>sixth item</item>
    </level2>
  </level1>
</root>

To visualise how the preceding-sibling axis works, I wrote the following
XSLT:

<xsl:template match="root">
  <xsl:variable name="item_list" select="//item"/>  <html>
  <body>
    <xsl:for-each select="$item_list">
      <p><b><xsl:value-of select="."/></b><br/>
        <xsl:choose>
          <xsl:when test="count(preceding-sibling::*) &lt; 1">No preceding
siblings<br/></xsl:when>
          <xsl:otherwise>Preceding siblings are:
            <xsl:for-each select="preceding-sibling::*">
              <xsl:value-of select="."/>&#160;
            </xsl:for-each>
          </xsl:otherwise>
        </xsl:choose>
      </p>
    </xsl:for-each>
  </body>
  </html>
</xsl:template>

I first create a variable, item_list, to contain all item elements. I then
visit each element in this list, displaying both it and its preceding
siblings.

What I would expect to see as output is:

first
No preceding siblings

second
Preceding siblings are: first

third
Preceding siblings are: first second

fourth
Preceding siblings are: first second third

fifth
Preceding siblings are: first second third fourth

sixth
Preceding siblings are: first second third fourth fifth


What I actually see though is:

first
No preceding siblings

second
Preceding siblings are: first

third
Preceding siblings are: first second

fourth
No preceding siblings

fifth
Preceding siblings are: fourth

sixth
Preceding siblings are: fourth fifth


Why is this so?

Once I'd created the item_list variable and used this for the <xsl:for-each>
I expected all item elements to have the same ancestor and therefore all be
siblings of each other. The results suggest that the original document
hierarchy is being used instead.

Am I missing something really obvious here?!

Thanks,

Darren Scott


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