<xsl:template match="/">
 <table frame="void" cellpadding="5" cellspacing="3" rules="cols">
 <tr>
  <xsl:for-each select="issues/issue">
    <xsl:variable name="curTheme" select="./theme_is"/> 
    <xsl:if test="not(preceding-sibling::issue[theme_is = $curTheme])"> 
      <td><xsl:value-of select="$curTheme"/></td>
    </xsl:if>
 </xsl:for-each>
 </tr>
 </table>  
</xsl:template>