allocate chunk of sequence

hello,

I try to allocate a chunk of ids from a sequence with the following
proc. However, if I don't use the 'lock lock_table', the proc may not
work when it runs at the same time by different psql sessions. Is there
a better way without using the 'lock lock_table' ?

Thanks,
Gary

create or replace function proc_allocate_seq(int)
returns int as $$

declare
nNumberOfFiles alias for $1;

aFileId int;
aNewFileId int;

begin

lock lock_table;

aFileId = nextval('aa_seq');

; sleep(3); if you have the proc

aNewFileId = setval('aa_seq', aFileId + nNumberOfFiles - 1);

return aFileId;

end;
$$ language plpgsql;