(If, Then, Else) LOGIC in XSL

Syntax:
<xsl:if test="XXX">
<!-- do something -->
</xsl:if>

<xsl:choose>
<xsl:when test="XXX">
<!-- do one thing -->
</xsl:when>
<xsl:when test="YYY">
<!-- do another thing -->
</xsl:when>
<xsl:otherwise>
<!-- do default thing -->
</xsl:otherwise>
</xsl:choose>


Logic examples:
<xsl:when test="matches(sql:feedurl_id, '^(\d+)$')">
<xsl:if test="$canonical-urls/agg:canonical-url[text()=$canonical-link-href][1]/@module = $module-type">
<xsl:when test="$module-type = 'knowledge'">
<xsl:when test="$module-type='iplayer' or $module-type='schedule'">
<xsl:if test="$debug">
<xsl:when test="document('')//this:modules/this:module[@type=$module-type]/this:merge-feeds">
<xsl:if test="position()=1">
<xsl:if test="position()=3 and count(agg:module)=0">
<xsl:when test="starts-with($name,'profile') or starts-with($name,'weather')">
<xsl:when test="starts-with(@type,'profile')">
<xsl:when test="@type='weather'">
<xsl:when test="count($modules/agg:modules/agg:module[starts-with(@type,'profile')])>0">