How about:

<table>
  <xsl:for-each-group select="//data/a" group-starting-with="a[@col-nbr
lt preceding-sibling::a[1]/@col-nbr]" >
    <xsl:variable name="row" select="position()" />
    <xsl:for-each select="current-group()" >
      <b col-nbr="{@col-nbr}" row-nbr="{$row}" />
    </xsl:for-each>
 </xsl:for-each-group>
</table>

Brad.