Conditional xsl:use-attribute-sets

Am I doing something wrong because this is not working:

        <fo:table-row hyphenate="true">
            <xsl:if test="position() mod 2 = 0">
                <xsl:attribute
name="xsl:use-attribute-sets">background-gray</xsl:attribute>
            </xsl:if>

Is that the correct way to call an attribute set conditionally?