filtering after join - Mailing list pgsql-sql

I want to use a UDF to filter tuples t that are generated after a join.
More specifially, I have a UDF foo(record), which computes a value for
a given tuple.  I can do the filtering before the join. e.g.:

select * from A, B where foo(A)<2 and A.a=B.b;

But I want to apply foo() to the tuples generated by the join
operation. How can I do that?
Thanks!

--
andrew