automatic incrementation

hello,

I have a table with the field 'id' which i would like to increment at
each insert automatically.
The idea is to create a function which determines the max(id) and then a
rule to insert instead
the real insertion instruction, with the next id value.

-- table
create table toto (
id int4 primary key,
project char(8) not null,
comments text
);
insert into toto values (1,'PTest','ndfjSHJG');
insert into toto values (2,'PTest','ndfjSHJG');
insert into toto values (3,'PTest','ndfjSHJG');

-- function
create function nextid(toto) returns int4
as ' select max($1.id)+1 as id ; '
language 'sql';

**** ERROR: type id lookup of 12 failed
This is my first problem

-- rule
create rule r_id as on insert to toto
do instead
insert into toto (id, project, comments) values (nextid(),
new.project, new.comments);

**** The next problem may be the rule writing !!!
I would like to have , even some other suggestions to make it works.
I can't use a sequence table because I've already one in the database.

Thanks,
Nuch