Why does this work?

Hi,
I have this XML data:

<rowset>
 <row>
  <customer>1</customer>
  <account>11</account>
 </row>
 <row>
  <customer>1</customer>
  <account>22</account>
 </row>
 <row>
  <customer>2</customer>
  <account>33</account>
 </row>
  <customer>2</customer>
  <account>44</account>
 </row>
</rowset>

I want to get output like this:

<table>
 <tr>
  <td>1</td>
  <td>11</td>
 </tr>
 <tr>
  <td>&nbsp;</td>
  <td>22</td>
 </tr>
 <tr>
  <td>2</td>
  <td>33</td>
 </tr>
 <tr>
  <td>&nbsp;</td>
  <td>44</td>
 </tr>
</table>

I solved it with the following template (this assumes that the data is
ordered by customer):

<xsl:template match="row">
 <xsl:choose>
  <xsl:when test="customer = preceding::row[position()=1]/customer">
   <td>&nbsp;</td>
  </xsl:when>
  <xsl:otherwise>
   <td><xsl:value-of select="customer"/>
  </xsl:otherwise>
 </xsl:choose>
 <td><xsl:value-of select="account"/></td>
</xsl:template>

Now I'm glad it works, but for some peace of mind I would like to know why 
<xsl:when test="customer = preceding::row[position()=1]/customer"> 
behaves as it does?


Thanks in advance,
Christian Schmitt


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