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