getting trigger to work

This function & trigger work:

create function prosp_tf_bi() returns opaque as '
begin
if new.pros_id is null then
new.pros_id:= 1000;
end if;
return new;
end; ' language 'plpgsql';

create trigger prosp_tr_bi before insert on prospector
for each row execute procedure prosp_tf_bi();

This function & trigger don't work:

create function prosp_tf_bi() returns opaque as '
begin
if new.pros_id is null then
new.pros_id:= A1000;
end if;
return new;
end; ' language 'plpgsql';

create trigger prosp_tr_bi before insert on prospector
for each row execute procedure prosp_tf_bi();

The error is "Attribute 'a1000' not found"

The column pros_id is char(5).
How do I put in the 'A1000'?