traversing the tree from an arbitrary node

I am trying to traverse the tree from an arbitrary node (as designated by
its ID).  But I can't get it right!

As some background, I have an XML document where if I find a certain tag
with a certain attribute I need to check the previous and/or following
identical tag to see if it is nested at the same depth within the document.
If not, there is a basic structural error within the file that needs to be
corrected before the XML file can be fully processed.

Here is some sample XML:

<rootTag>
  <parentTag>
    <childTag id='id1' att='checkThis'>
      . . . some info . . .
    </childTag>
  </parentTag>
  <parentTag>
    <childTag id='id2' att='thisChecks'>
      . . . some more info . . .
    </childTag>
  </parentTag>
</rootTag>

When I find a <childTag>, I count how deep the tag is from <rootTag>, get
the ID of the next <childTag>, then try to see if the next <childTag> is at
the same depth.  Here are the steps from the stylesheet:

<xsl:if test="@att='checkThis'">
  <xsl:variable name="elementDepth">
    <xsl:value-of select="count(ancestor::*)" />  <!-- this gets the depth
of the current childTag from rootTag, right? -->
  </xsl:variable>
  <xsl:variable name="id">
    <xsl:value-of select="following::childTag[1]/@id" />
  </xsl:variable>
  <xsl:choose>
    <xsl:when test="id('$id')/from-ancestors(*[position()
=$elementDepth][self::rootTag])">
      <!-- do nothing since this is the right depth -->
    </xsl:when>
    <xsl:otherwise>
      . . . error message . . .
    </xsl:otherwise>
  </xsl:choose>
</xsl:if>


I am using XT on a Windows NT platform.  When I run this, I get the error
message of "expected node test" for the line that has the id() function.
What I am trying to say for that node test is "from the node that has this
id, go up $elementDepth ancestor nodes and see if that node is <rootTag>."
What am I doing wrong?

Thanks,
Chris

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Chris d'Aquin, XML Engineer
Questia Media, Inc.
713.358.2515
cdaquin@xxxxxxxxxxx
http://www.questia.com/
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