workday function

Hi folks

I need to be able to add and subtract workdays, something like

select CURRENT_DATE - '3 work days'::interval;

I can't see how to do this natively so I'm looking to write a function to do 
it and was wondering if anyone's already done it.

While Googling I've found that MS Excel has a workday function which seems to 
do what I want.

Any help would be appreciated.
-- 
Gary Stainburn
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