XSLT - Adding a class to something with a class?

Question

When using XSLT how do I apply a class to an element which already has a class? The way I'm doing it it replaces the class that is already present? How would I add the class in addition to the existing class? My code is as follows:

<xsl:if test="data[@alias = 'off'] = 1">
    <xsl:attribute name="class">off</xsl:attribute>
</xsl:if>
<xsl:if test="$currentPage/ancestor-or-self::node/@id = current()/@id">
    <xsl:attribute name="class">active</xsl:attribute>
</xsl:if>

Thanks.

Answer 1

The other way around:

<xsl:attribute name="class">
  <xsl:if test="data[@alias = 'off'] = 1">off </xsl:if>
  <xsl:if test="$currentPage/ancestor-or-self::node/@id = current()/@id">active </xsl:if>
</xsl:attribute>

Note the extra space I put after every attribute value. The XSLT processor will trim the trailing space from the attribute value on its own, so no need to do complicated space handling.