Yet Another Grouping Question

Input XML fragment:

<list>
    <item><prog>bar</prog><version>2</version></item>
    <item><prog>bar</prog><version>3</version></item>
    <item><prog>foo</prog><version>1.2</version></item>
    <item><prog>foo</prog><version>1.3</version></item>
    <item><prog>xyzzy</prog><version>1.0</version></item>
</list>

Desired output:

HTML table with one row per item, sorted by program name.  All `bar'
lines have green background, all `foo' lines have yellow background,
all `xyzzy' lines have green background, and so on.

I.e.: change background color when prog element changes.

Thanks to the FAQ http://freespace.virgin.net/b.pawson/xsl I think I
know how to create output when the prog element changes, but how to
toggle between green and yellow background?  This would mean I need a
variable which contains the current background color, but since
variable assignment is not possible in XSLT...

tia,
kai
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