A simple join question that may stump you - Mailing list pgsql-sql

OK, I have 2 tables, table A:
  ID  FLAG
----- -----   1     1   2     1   2     2   3     1   3     2   3     3 
and table B:
FLAG
-----   1   2

I want to find all id's from table A that have every flag in table B
but no extra flags.  So, I'd end up with:
  ID
-----   2

As id 2 has both flag 1 and flag 2, id 1 doesn't have flag 2, and id 3
has flag 3.

I know it can be done, 'cause I've done it in the past, but I've spent
hours on this to no avail.  Surfing the net proved fruitless as well.

Any help would be greatly appreciated.