<items>
   <product>x</product>
   <product>y</product>
   <product>z</product>
   <product>a</product>
</items>

In order to avoid starting and ending with a comma I have been
struggling with a template match like this:

   <xsl:for-each select="product">
             <xsl:if test="not(position() = first() | last())">
                 <xsl:text>,  </xsl:text><xsl:value-of select="node()"/>
             </xsl:if>
         </xsl:for-each>