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Other XPath Functions
index-of(sequence,itemvalue)
Returns position in sequence where itemvalue is
string-join(sequence,delimiter)
Returns sequence separated by the delimiter 
<colors><color>red</color><color>blue</color><color>green</color></colors>

<xsl:template match="colors">
 <xsl:value-of select="string-join(color,',')"/>
</xsl:template>

red,blue,green