<xsl:choose>
      <xsl:when test="not(section)"><xsl:text>none</xsl:text></xsl:when>

if you get past that not(section) is false so  the following test is
   not(false() and not(title))
so it is always true

      <xsl:when test="not(not(section) and
not(title))"><xsl:text>all</xsl:text></xsl:when>

so you can not reach here
      <xsl:otherwise><xsl:text>mixed</xsl:text></xsl:otherwise>
    </xsl:choose>