Grouping By Key (Code)
<xsl:for-each-group select="cities/city"
group-by="@country">
<tr>
<td><xsl:value-of select="position()"/></td>
<td><xsl:value-of select="@country"/></td>
<td>
<xsl:value-of select="current-group()/@name"
separator=", "/>
</td>
</tr>
</xsl:for-each-group>