Calculation using the position of an element??

Hi all, 

> Ive tried to come up with an XPath solution involving position()  (one
> line)  to produce a certain value using <xsl:value-of select="......."/>
My Sample XML File

> <tgroup cols = "5">
>       <colspec colname="col1"/>
>       <colspec colname="colspec1"/>
>       <colspec colname="col2"/>
>       <colspec colname="col4"/>
> </tgroup>
>
> I have two different values (say x and y) elsewhere in the xml doc and I
> need to perform a small calculation that I will now outline in
> Pseudocode...
>
> If x="colspec1" then
>       position1 = 2
> If y="col4" then
>       position2 = 4
> Result= position2 - position1
>
> ie Result=2
>
>
> Can anyone help?
>
> Thanks
> Karen
>       
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