[xsl] Restrictind a set of notes to within a group

["Mark" <mark@xxxxxxxxxxxx>]

have an xml file like
<Month>
   <Date day=b1b>
       <Session task=btask1b>
           <Notes hours-worked=b1.5b/>
       </Session>
       <Session task=btask2b>
           Notes hours-worked=b5b/>
       <Session task=btask1b>
           <Notes hours-worked=b3b/>
       </Session>
   </Date>
   <Date day=b2b>
       ....
   </Date>
</Month>
For the month, I want to sum all the hours-worked for each task:

I have tried several expressions, but failed. This gives me the total hours 
worked on all tasks rather than each individual task:
<xsl:for-each-group select="Date/Session" group-by="@task">
     <xsl:for-each select=".">
       <fo:block xsl:use-attribute-sets="subdiv2">
         <xsl:value-of select="@task"/>
         <xsl:text> (</xsl:text>
         <xsl:value-of 
select="format-number(sum(../../Date/Session/Notes/@hours-worked), 
'.00')"/>
         <xsl:text>) </xsl:text>
       </fo:block>
   </xsl:for-each>
</xsl:for-each-group>
How do I restrict the total to just the hours-worked on each specific task?
Thanks,
Mark