Calculations in XSL

I need to add two values togethor, both are attributes.  I have seen several
examples, all which look like they should work fine, but all of them have
given me this error:

Keyword xsl:variable may not be used here.

Are there circumstances where xsl:variable cannot be used?  I'm using it
like this:

<xsl:variable name="sum" select="$a + $b"/>

If anyone knows of an example that works please could they send me the
script.

cheers alex


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