Insert into a table with only a SERIAL

I am pretty much self taught using SQL, and I suspect that my problem
here is trying to do something silly. I have since changed my tables
and avoided the problem, but I am curious as to why this happens, and
it's remotely possible I have even found a bug or two.

I created a table with only a sequence:

CREATE TABLE aaa (
id SERIAL,
);

I can't insert into aaa unless I pass a value; these don't work:

insert into aaa;
insert into aaa values ();
insert into aaa () values ();

But this does:

insert into aaa values (1);

except that the value inserted is not known to the sequence.
(** This seems a bug to me **)

This also works:

insert into aaa values (nextval('aaa_id_seq'));

but sort of defeats the purpose of using a type SERIAL rather than
hand rolling my own sequence.

--
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Felix Finch: scarecrow repairman & rocket surgeon / felix(at)crowfix(dot)com
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I've found a solution to Fermat's Last Theorem but I see I've run out of room o