[Ur] --- operator

[Marc Weber]

Although I fully understand what it does I still don't know how .ur code
looks like which uses --- operator.

Can someone just replace the (* now drop AA .. *) comment by working
code so that I can learn how this is done?
In which way would the existing typing rule have helped me figuring out
what to place there?

        let
          val rc1rc2 = {AA = 7, B = 8}

          val r = rc1rc2 --- (* now drop AA using --- operator *)

          (*  using -- would be:  rc1rc2 -- #AA *)

        in
          r.B
        end

Thanks
Marc Weber