[xsl] Doing math in XSLT?

["dvint@xxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

I have the following XML

<row>
	<entry nameend="col2" namest="col1">
		<p>OpenID Connect client settings</p>
		<note
			id="note_N1055C"
			>The following parameters are only applicable
			when this client supports the OpenID Connect use
			cases.</note>
	</entry>
</row>

The attributes nameend="col2" namest="col1" indicate that this row spans 
2 columns (end - start + 1).

I've tried this to try and get the numbers 1 and 2 from these strings, 
but I get NaN errors

<xsl:variable name="start"
	select="number(substring-after(@namest, 'col'))" as="xs:double"/>
<xsl:variable name="end"
	select="number(substring-after(@nameend, 'col'))" as="xs:double"/>

I've tried this in different combinations with/without the 
as="xs:double" and number() and those variations throw the same error.

The table itself has this information

<table colsep="0" frame="none" id="table_post_oauthClients_endpoints"
	rowsep="0">
	<tgroup align="left" cols="2">
		<colspec colname="col1" colnum="1" colwidth="1*"/>
		<colspec colname="col2" colnum="2" colwidth="3*"/>
		<thead>
			<row>
				<entry colname="col1">Parameter</entry>
				<entry colname="col2">Description</entry>
			</row>
		</thead>

I'll try referencing the <colspec> element and using the @column value, 
but that seems like it will give me the same issue.

..dan