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'?