Doing math in XSLT?

<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>

<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"/>

<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>