A little more thinking reveals that:

<xsl:choose>
  <xsl:when test="position() = 1">
    <tr>
      <td><xsl:value-of select="."/></td>
      <td>1</td>
      <td><xsl:value-of select="@count"/></td>
    </tr>
  </xsl:when>
  <xsl:otherwise>
    <tr>
      <td><xsl:value-of select="."/></td>
      <td><xsl:value-of select="sum(preceding::reg/@count) + 1"/></td>
      <td><xsl:value-of select="sum(preceding::reg/@count) + 
@count"/></td>
    </tr>
  </xsl:otherwise>
</xsl:choose>


can be just:

<tr>
  <td><xsl:value-of select="."/></td>
  <td><xsl:value-of select="sum(preceding::reg/@count) + 1"/></td>
  <td><xsl:value-of select="sum(preceding::reg/@count) + @count"/></td>
</tr>

because the sum of the empty set will be 0.

Jay Bryant
Bryant Communication Services
(presently consulting at Synergistic Solution Technologies)