Thank you again,
ac

Hmm, I usually reply to messages very early in the morning, possibly
sometimes feeling sleepy.

Just change in my code:

<xsl:key name="kChildren" match="section"
  use="generate-id(preceding::section
                        [@level = current()/@level - 1]
                        [last()]
                   )"
/>

With:

<xsl:key name="kChildren" match="section"
  use="generate-id(preceding::section
                        [@level = current()/@level - 1]
                        [1]
                   )"
/>

and now you'd be getting the correct results.

Hope this helped.