[xsl] simple choose : when problem

Hi all

This is another choose:when problem.  Just to test the validity of my
choose:when I written the following xslt segment.
I would expect the output of this segment be MileMay. But to my surprise its
M3.

<xsl:variable name='unit' select="mm"/>
<xsl:choose>
	<xsl:when test="$unit='mm'">
		MileMay
	</xsl:when>
	<xsl:when test="$unit='cm'">
		Count Mile
	</xsl:when>
	<xsl:otherwise>M3</xsl:otherwise>
</xsl:choose>

What have I done wrong here?

rgds
joe

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