Thnaks for all the replies I got on this subject - they have clarified my
thoughts somewhat

-- 
Louise Renton
Bone Research Unit
University of the Witwatersrand
Johannesburg
South Africa
"There are nights when the wolves are silent and only the moon howls".
George Carlin
No trees were killed in the sending of this message.
However, many electrons were terribly inconvenienced.
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