serial drop error

The following queries result in a dropped sequence, but IMO should not:

create table foo(id serial);
create table bar(id integer not null nextval('foo_id_seq'::text));
alter table foo alter column id drop default;
drop table foo;

Once dependence between foo and foo_id_seq has been removed, a drop of foo
should not drop foo_id_seq, particularly if someone else is using it as a
default.  This occurs in 7.3.4 and 7.4.6.

Ed