partial unique constraint

Trying to come up with the proper syntax to meet the following criteria:

create table foo (bar integer, baz boolean UNIQUE (bar, baz = true)); 

note the above syntax is not correct, but should demonstrate what i'm
trying to do; I want to add a unique constraint such that we only allow
one case of bar and baz = true...  i can have unlimited bar and baz =
false, and there can be multiple bar and baz = true if the bars are
different...  did some doc reading and mail list searching but a valid
syntax for this seems to be escaping me... 

btw I'm pretty sure I could do this with an external trigger, but am
wondering about a constraint oriented approach

Robert Treat
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