<xsl:template match="catalog">
    <head>
       <title><xsl:value-of select="category"/></title>      
    </head>
    <body>
       <h1><xsl:value-of select="category"/></h1>      
       <xsl:apply-templates select="composer">
         <xsl:sort select="name/last_name"/>
       </xsl:apply-templates>
       <hr/>
       Copyright <xsl:value-of select="copyright"/><br/>
       Last Modified: <xsl:value-of select="last_updated"/><br/>
       <xsl:apply-templates select="maintainer"/>
    </body>
  </xsl:template>
The select attribute provides the key to sort by
Must be a child of xsl:apply-templates or xsl:for-each