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>


 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list