A more concise way to handle empty numeric elements

Hi All

I have to process an XML file that contains a lot of numeric data. Many of the
elements that have no value are empty instead of being zero and it may differ
for each line, for example

<line id=1>
            <cost1>1000</cost1>
            <cost2>0</cost2> <!-- 0 here, but empty in next line -->
            <cost3></cost3>
</line>
<line id=2>
            <cost1>2000</cost1>
            <cost2></cost2>
            <cost3>3000</cost3>
</line>

I want to do:

<xsl:for-each select="line">
       <xsl:value-of select="format-number(cost1 + cost2 + cost3,'#,###')"/>
      </xsl:for-each>

However, the empty elements will cause "NaN" to be displayed

To fix it I can do:

<xsl:for-each select="line">
<xsl:variable name="cost1">
      <xsl:choose>
            <xsl:when test="(cost1 &gt;= 0)"><xsl:value-of
select="cost1"/></xsl:when>
            <xsl:otherwise>0</xsl:otherwise>
      </xsl:choose>
</xsl:variable>
<xsl:variable name="cost2">
      <xsl:choose>
            <xsl:when test="(cost2 &gt;= 0)"><xsl:value-of
select="cost2"/></xsl:when>
            <xsl:otherwise>0</xsl:otherwise>
      </xsl:choose>
</xsl:variable>
<xsl:variable name="cost3">
      <xsl:choose>
            <xsl:when test="(cost3 &gt;= 0)"><xsl:value-of
select="cost3"/></xsl:when>
            <xsl:otherwise>0</xsl:otherwise>
      </xsl:choose>
</xsl:variable>
       <xsl:value-of select="format-number($cost1 + $cost2 +
$cost3,'#,###')"/>
      </xsl:for-each>

However, it seems very long winded. Also the output is HTML, so it makes it
hard to read the HTML with so much XML interwoven.

I realized I could write a template to return 0 for each empty element, but
the call-template and parameter declaration will add further lines.

Is there a more succinct/elegant way to do it? (it's a shame "number(cost1)"
doesn't return 0 for empty elements)

BTW: I'm using XSLT1.0 and can't use any extensions

Thanks in advance
M a r k