A variable in the test attribute of <xsl:if> ?

This may well be impossible, but I was wondering if (using MSXSL) I
could somehow use a variable in the test attribute of <xsl:if> ?

Instead of <xsl:if test="foo = 'bar'">, I would like to have "foo =
'bar'" in a variable and do this:

    <xsl:if test="$test">

Every time I search, I find mentions of the jscript dyn:evaluate
function:

 function evaluate(context, expression)
      {
         return context.nextNode().selectNodes(expression);
      }


 but that only seems to work in returning a value -- that is, I can do:

   <xsl:value-of select="dyn:evaluate(., <SOME EXPRESSION HERE> )"/>

but I can't do

    <xsl:if test="dyn:evaluate(., <SOME EXPRESSION HERE> )">

to return a boolean for whether the node matches.

Is there another function I could use?

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