dimension arithmetic in xpath

I want to have an option in my stylesheets to make figures
automatically scale to the width of the page. frustrated in XSL FO
(see yesterdays email, Nikolai convinced me I misread the spec), I
try to do it in the XSLT part. What I would _like_ to say is

      <xsl:attribute name="content-width">
          <xsl:value-of select="$pageWidth - $PageMarginLeft - $PageMarginRight"/>
      </xsl:attribute>

ie set the width to the pagewidth minus the margins (these are
variables containing eg 211mm and 60pt). but of course that does not
work (since the things are not numbers).

can anyone see a way I can get this to work?

Sebastian



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