support for @hidden-when-empty

tree.xsl

···
   <xsl:template match="f:backmatter//f:backmatter">
   </xsl:template>
 
-  <xsl:template match="f:tree">
+  <xsl:template match="f:tree[f:mainmatter[*] or not(@hidden-when-empty = 'true')]">
     <section>
       <xsl:attribute name="lang">
         <xsl:choose>
···
 
     <xsl:apply-templates select="f:backmatter" />
   </xsl:template>
+
+  <xsl:template match="f:tree"></xsl:template>
+
+
 
 </xsl:stylesheet>