test conditions (xsl:when) involving constants using saxon

I am using saxon 4.1 and I 
am trying to use constants with 
a test condtion as follows:

<xsl:choose>
   <xsl:when test="[{constant(partyid)} = '0']">
	 .... do stuff....       
   </xsl:when>
   <xsl:otherwise>
 	.... do other stuff....                 
   </xsl:otherwise>
</xsl:choose>

I get the following error: 
    Failed while processing  input document: org.xml.sax.SAXException:
Error in pattern [{constant{partyid)} = '0']:  Unexpected token [ boeyond
end of pattern.


This is using the saxon notation of a constant.
I am not sure that constant evaluations are supported
inside of test conditions.  Has anyone else tried
something like this?  Is there a work around that I
am not seeing?  

I am using Saxon to as it enables me to define my
own tags, which I am using until more parsers 
support extension functions.  So switching parsers
is not an option right now (I got XT extension functions
working, but had to find and fix a bug first, so I am waiting
for it to stablize).


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