Alternating row background color - a twist

The typical way of alternating the background color of every other row
in an HTML table is something like this:

<xsl:for-each select="msxsl:node-set($sorted_legislators)">
	<tr>
		<xsl:if test="position() mod 2 = 0">
			<xsl:attribute
name="bgcolor">#eeeeee</xsl:attribute>
		</xsl:if>

------------------------------------

My situation is this - the for-each is based on legislators sorted by
district - here's a sample (not real) xml to illustrate:

<legislators>
    <legislator>
         <name>Joe</name>
         <district_id>1</district_id>
    </legislator>
    <legislator>
         <name>Bob</name>
         <district_id>2</district_id>
    </legislator>
    <legislator>
         <name>Pete</name>
         <district_id>2</district_id>
    </legislator>
</legislators>

There will be one *or more* legislators for each district.  Instead of
alternating the background color for every *legislator*, I want to
alternate the background color for every *district*.  In other words, I
want Joe in district_id 1 to have white background, but I want both Bob
and Pete in district_id 2 to have a gray background.  But since I can't
change the value of a variable (can't increment a counter), I don't know
what I can base the bgcolor alternation on.

The xsl:for-each is based on a select of a variable containing all
legislators sorted by district.

I know there is probably an easy way to do this, but I don't know it. 
Help!

Thanks.

Hardy Merrill