XSL for-each: how to detect last node?

I have this simple code:

<xsl:for-each select="GroupsServed">
  <xsl:value-of select="."/>,<br/>
</xsl:for-each></font>

I'm trying to add a comma for each item added.

This has 2 flaws:

1. Case of when there's only 1 item: the code would unconditionally add a comma.
2. Case of when there's more than 1 item: the last item would have a comma to it.

What do you think is the most elegant solution to solve this?

I'm using XSLT 2.0

If you're using XSLT 2.0, the canonical answer to your problem is

<xsl:value-of select="GroupsServed" separator=", " />

On XSLT 1.0, the somewhat CPU-expensive approach to finding the last element in a node-set is

<xsl:if test="position() = last()" />

Final answer:

<xsl:for-each select="GroupsServed">
  <xsl:value-of select="."/>                                    
  <xsl:choose>
    <xsl:when test="position() != last()">,<br/></xsl:when>
  </xsl:choose>
</xsl:for-each>

<xsl:variable name="GROUPS_SERVED_COUNT" select="count(GroupsServed)"/>
<xsl:for-each select="GroupsServed">
    <xsl:value-of select="."/>
    <xsl:if test="position() < $GROUPS_SERVED_COUNT">
        ,<br/>
    </xsl:if>
</xsl:for-each></font>