Anongst other stuff my input documents contain sections, and sections can
feature as descendants of other sections (either as children or as lower
order descendants). Certain sections are marked with a particular attribute
which I described as "significant".

I think the key that I missed out is how this significance affects later
processing of these documents: in one stylesheet which runs across these
documents any sections with the significant attribute invoke a new
result-document.

The condition which I am trying to detect is where there will be content
"left over". I suppose you could think of it as a place where the tree model
and the linear model are in conflict.
...
This case is not OK, the .D. section region is left hanging when the F2
result document is completed and thus it goes back into F1.

All the sections have ids (which I've left out of the example) which can be
used to report a problem.

t:\ftemp>type trevor1.xml
  <!-- F1 -->
    <section>
      <include>
        <section>.A.</section>
  <!-- F2 -->
          <section sig="Y">
            <section>.B.</section>
            <include>
              <section>.C.</section>
            </include>
          </section>
  <!-- F3 -->
          <section sig="Y">
            <section>.D.</section>
          </section>
        </include>
      </section>

t:\ftemp>type trevor2.xml
  <!-- F1 -->
    <section>
      <include>
        <section>.A.</section>
  <!-- F2 -->
        <section sig="Y">
          <section>.B.</section>
          <include>
            <section>.C.</section>
          </include>
        </section>
  <!-- F1 -->
        <section>
          <section>.D.</section>
        </section>
      </include>
    </section>

Problem section:
t:\ftemp>type trevor.xsl
<?xml version="1.0" encoding="US-ASCII"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                version="1.0">

<xsl:template match="/">
  <xsl:for-each select="//section[not(@sig='Y')]
                                 [preceding-sibling::section/@sig='Y']">
Problem section: <xsl:value-of select="@id"/>
  </xsl:for-each>
</xsl:template>

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