select the attribute II, compare the atrribute

i have, in one XML file this:
<id_mensajesforos seleccionado="si">46</id_mensajesforos>

how can i determine the attribute value?
for this i am making this:
xsl:choose>
<xsl:when test="id_mensajesforos[@seleccionado='si']">
s </xsl:when>
<xsl:otherwise>n</xsl:otherwise>
</xsl:choose>

but it doesn't run well.
can you help me for knowing if the seleccionado attribute value is "si or
no?
thanks



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