[xsl] Two versions of sum over node list by recursion--why and how does

Here are two examples of summing a node list by recursion: the first sends 
the result of the addition back to itself by a parameter: the second does 
not. The first was posted by G. Ken Holman--the second is from Michael Kay 
s _XSLT Programmer's Reference_. 

I know that Mr. Kay's code works...I just can't figure out _why_. 

FIRST EXAMPLE:

<xsl:template match="/">
  <xsl:variable name="result-text">
    <xsl:text/>0<xsl:apply-templates select="/*/RECEIPT[1]"
mode="equation"/>
  </xsl:variable>
  <xsl:variable name="result" select="number($result-text)"/>

  <xsl:value-of select="$result"/>
</xsl:template>

<xsl:template match="RECEIPT" mode="equation">
  <xsl:param name="result" select="0"/>
  <xsl:variable name="this" select="$result + ( qtyRcpt * line/pr )"/>
  <xsl:choose>
    <xsl:when test="following-sibling::RECEIPT">
      <xsl:apply-templates select="following-sibling::RECEIPT[1]"
                           mode="equation">
        <xsl:with-param name="result" select="$this"/>
      </xsl:apply-templates>
    </xsl:when>
    <xsl:otherwise>
      <xsl:value-of select="$this"/>
    </xsl:otherwise>
  </xsl:choose>
</xsl:template>

</xsl:stylesheet>

SECOND EXAMPLE:

<xsl:stylesheet 
   xmlns:xsl="http://www.w3.org/1999/XSL/Transform";
   version="1.0">

<xsl:template name="total-sales-value">
   <xsl:param name="list"/>
   <xsl:choose>
      <xsl:when test="$list">
         <xsl:variable name="first" select="$list[1]"/>
         <xsl:variable name="total-of-rest">
            <xsl:call-template name="total-sales-value">
               <xsl:with-param name="list" select="$list[position()!=1]"/>
            </xsl:call-template>
         </xsl:variable>
         <xsl:value-of select="$first/sales * $first/price +
$total-of-rest"/>
      </xsl:when>
      <xsl:otherwise>0</xsl:otherwise>
   </xsl:choose>
</xsl:template>


<xsl:template match="/">   
   <xsl:variable name="total">
        <xsl:call-template name="total-sales-value">
            <xsl:with-param name="list" select="//book"/>
        </xsl:call-template>
   </xsl:variable>
Total sales value is: <xsl:value-of select="format-number($total,
'$#.00')"/>   
</xsl:template>
</xsl:stylesheet>

QUESTION:

In Mr. Kay's example, why does the variable $total-of-rest indeed contain
the total 
and not just the result of the addition in the last recursion?  



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