Constraint Problem

I want to create a constraint that's a little different, but don't know
how to implement it. I have an intermediate table with a compound PK and
a boolean 'ysnDefault' column:

   comanyID     INTEGER REFERENCES companies,
   assocationID INTEGER REFERENCES associations,
   ysnDefault     BOOLEAN

I just want to constrain the ysnDefault field to only allow one TRUE
value for any companyID/associationID pair, with no restrictions on the
number of FALSES.

At first glance I thought it would be easy, but I can't see how to do
it. Any suggestions?

Thanks
Ron