sum a selected tag of a key

Hi everybody and thanks for the latest response,

I have  created a key 
<xsl:key name="detalleanno" match="/PENSIONES/DETALLE_PENSION" 
use="/PENSIONES/DETALLE_PENSION/ANNO" />
I would like to get the sum of APORT_EMP during a year. I am trying with the 
following expression:
<xsl:value-of select="sum(key('detalleanno',ANNO)/APORT_TRAB)"/>, but i always 
get the sum of all the years and i only want the values of a year. Could anyone 
give me an advice?
thanks






<PENSIONES>
        <NOMBRE>28509599H</NOMBRE>
        <numCol>1</numCol>
        <Volver>0</Volver>
        <CIF>P4100000A</CIF>
        <RAZONSOCIAL>DIPUTACION PROVINCIAL DE SEVILLA</RAZONSOCIAL>
        <TOTAL_APORT_EMP>4.640,55</TOTAL_APORT_EMP>
        <TOTAL_APORT_TRAB>1.969,44</TOTAL_APORT_TRAB>
        <accion>listadoPensiones</accion>
        <desde>1</desde>
        <DETALLE_PENSION>
                <ANNO>1998</ANNO>
                <MES>4</MES>
                <APORT_EMP>1514.55</APORT_EMP>
                <APORT_TRAB>0</APORT_TRAB>
        </DETALLE_PENSION>
        <DETALLE_PENSION>
                <ANNO>1998</ANNO>
                <MES>5</MES>
                <APORT_EMP>42.07</APORT_EMP>
                <APORT_TRAB>8.41</APORT_TRAB>
        </DETALLE_PENSION>
        <DETALLE_PENSION>
                <ANNO>1998</ANNO>
                <MES>6</MES>
                <APORT_EMP>42.07</APORT_EMP>
                <APORT_TRAB>8.41</APORT_TRAB>
        </DETALLE_PENSION>
        <DETALLE_PENSION>
                <ANNO>1998</ANNO>
                <MES>7</MES>
                <APORT_EMP>43.07</APORT_EMP>
                <APORT_TRAB>8.41</APORT_TRAB>
        </DETALLE_PENSION>
        <DETALLE_PENSION>
                <ANNO>1998</ANNO>
                <MES>8</MES>
                <APORT_EMP>42.07</APORT_EMP>
                <APORT_TRAB>8.41</APORT_TRAB>
        </DETALLE_PENSION>
        <DETALLE_PENSION>
                <ANNO>1999</ANNO>
                <MES>9</MES>
                <APORT_EMP>42.07</APORT_EMP>
                <APORT_TRAB>8.41</APORT_TRAB>
        </DETALLE_PENSION>
        <hasta>70</hasta>
        <registros>71</registros>
</PENSIONES>


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