declare
        v_parent_sk     varbit default null;
        v_max_value     integer;
        v_rec           record;
        clr_keys_p      boolean default 't';
begin
        if new.message_id = old.message_id and 
           ((new.reply_to = old.reply_to) or 
            (new.reply_to is null and old.reply_to is null)) then

           return new;

        end if;

        for v_rec in select message_id, reply_to
                     from acs_messages
                     where tree_sortkey between new.tree_sortkey and tree_right(new.tree_sortkey)
                     order by tree_sortkey
        LOOP
            if clr_keys_p then
               update acs_messages set tree_sortkey = null
               where tree_sortkey between new.tree_sortkey and tree_right(new.tree_sortkey);
               clr_keys_p := 'f';
            end if;
            
            select max(tree_leaf_key_to_int(tree_sortkey)) into v_max_value
              from acs_messages
              where reply_to = v_rec.reply_to;

            select tree_sortkey into v_parent_sk 
              from acs_messages 
             where message_id = v_rec.reply_to;

            update acs_messages
               set tree_sortkey = tree_next_key(v_parent_sk, v_max_value)
             where message_id = v_rec.message_id;

        end LOOP;

        return new;

end;