partial uniqueness

There recently was a discussion on how to enforce
uniqueness on a row for a given condition, say allow many
address rows for a person where active = false but only one
where active = true. I cannot find it in the archives. Can
someone please point me to the messages so I can read up ?
Thanks. Also I can't find the relevant section in Celko's SQL
for Smarties anymore :-((  I know it's in there, I did see it
before...

I am trying to do it with triggers and a partial index
but can't seem to get there.

Can post details if wanted.

Thanks again,
Karsten
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