Adding Several Numerical Attribute Values into One At

Hi,

I'm trying to translate an xml table into html and I want to offset the
table head with a horizontal rule that spans the entire width of the
table.
So I need to generate a colspan attribute value that consists of all
other colspan attribute values.
If the colspan attribute is missing or "NaN", add one to the value.
I also have to use XML 1.0.

For example:

<tr>
	<th colspan="1"></th>
	<th colspan="2"></th>
	<th colspan="NaN"></th>
</tr>

I would need a colspan attribute that contains the value of 4.

I know that variables and parameters in XSLT don't work the same as a
coder is used to. I've looked at a few examples that simulate an
incrementing variable through a parameter so you can process a loop,
which I believe to be a similar situation to the one that I'm dealing
with, except I need to add a variant value depending on the attribute.

Here is an example of my XSL that doesn't work:

<xsl:template match="thead">
		<tr>
			<td>
				<xsl:attribute
name="colspan"><xsl:call-template name="get-col-span"/></xsl:attribute>
				<hr noshade="noshade"/>
			</td>
		</tr>
	<xsl:apply-templates/>
</xsl:template>

<xsl:template name="get-col-span">
	<xsl:apply-templates select="tr[1]" mode="build-col-span"/>
</xsl:template>

<xsl:template match="tr" mode="build-col-span">
	<xsl:param name="colspan_var"/>
	<xsl:call-template name="get-th">
		<xsl:with-param name="colspan_var">0</xsl:with-param>
	</xsl:call-template>
</xsl:template>

<xsl:template match="th" name="get-th">
	<xsl:choose>
		<xsl:when test="@colspan != ''">
			<xsl:choose>
				<xsl:when test="@colspan != 'NaN'">
					<xsl:param name="colspan_var"
select="$colspan_var + @colspan"/>
				</xsl:when>
				<xsl:otherwise>
					<xsl:param name="colspan_var"
select="$colspan_var + 1"/>
				</xsl:otherwise>
			</xsl:choose>
		</xsl:when>
		<xsl:otherwise>
			<xsl:param name="colspan_var"
select="$colspan_var + 1"/>
		</xsl:otherwise>
	</xsl:choose>

	<xsl:variable name="next" select="following-sibling::th[1]"/>
	<xsl:choose>
		<xsl:when test="!($next)">
			<xsl:value-of select="$colspan_var"/>
		</xsl:when>
		<xsl:otherwise>
			<xsl:call-template name="get-th">
				<xsl:with-param
name="colspan_var"><xsl:value-of
select="$colspan_var"/></xsl:with-param>
			</xsl:call-template>
		</xsl:otherwise>
	</xsl:choose>
</xsl:template>

I know there is a problem with how I'm setting up my parameter value in
the get-th template. I'm not even sure if I'm addressing this problem in
the proper manner. Any help would be greatly appreciated. Thanks!