[xsl] ancestor/subsequent descendant test

My input documents are allowed to contain nested sections. An optional
attribute marks out certain sections as significant. I want to detect a
situation in which a section which contains a descendant significant section
does not contain a subsequent INsignificant section (other than descendants
of any significant sections). It's complicated because the nested sections
are not (necessarily) immediate descendants of each other.

In the following short example:

 <section>
   <include>
     <section>.A.</section>
     <section sig="Y">
       <section>.B.</section>
       <include>
         <section>.C.</section>
       </include>
     </section>
   <!-- NB -->
     <section sig="Y">
       <section>.D.</section>
     </section>
   </include>
 </section>

The section .A. is OK because there isn't a "significant" section before it.

The section .D. is OK because it is within a "significant section", but if
the section it is wrapped in did not have the significant attribute set then
I want to report it, because it is INsignificant and a descendant of a
section which contains a prior significant section.

I'm a bit stuck with this template, which identifies the correct ancestor,
but doesn't know how to detect a descendant non-significant section which is
outside and subsequent to the section which is originally matched by the
template. It surely isn't as complex as I'm finding it, can anybody help
please?

 <xsl:template match="section[(_at_)sig="Y"]>
   <xsl:for-each select="ancestor::*[self::section][1]">
     <xsl:if test="   what goes here ???">
       <xsl:message>error</xsl:message>
   </xsl:for-each>
 <xsl:template>

Thank you very much

Cheers
Trevor



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