select question

[postgreql 7.2, linux]
I have a table T with columns run, wafer, and test:
T(run text, wafer int, test text)
Given a run and a set of wafers, I need the set of tests that match
*all* the specified wafers:

run wafer test
a 1 foo
a 2 foo
a 3 foo
a 3 bar

E.g.
Given run 'a' and wafers (1,3) I should get one row: foo, since only foo matches both 1 and 3.
Given run 'a' and wafers (3) I should get two rows: foo,bar, since both foo and bar match 3.

Is there some neat way to do this in a single query?

Puzzled,
George

--
I cannot think why the whole bed of the ocean is
not one solid mass of oysters, so prolific they seem. Ah,
I am wandering! Strange how the brain controls the brain!
-- Sherlock Holmes in "The Dying Detective"