anchor.diff:  
   let log_primops cfg f =
       if cfg.debug.primop then f () else ()
  +
  +let log_unfolding cfg f =
  +    if cfg.debug.unfolding then f () else ()
   
   let is_empty = function
       | [] -> true
  
positive.code:         Some (cfg, List.tl stack)
  
      | _ ->
          failwith <| BU.format1 "Unexpected unfolding result, (%s, %s, %s)" (string_of_res res)
  
  let rec norm : cfg -> env -> stack -> term -> term =
      fun cfg env stack t ->
  
negative.code:     | _, Some _, _, _
      | _, _, Some _, _
      | _, _, _, Some _ ->
          log_unfolding cfg (fun () -> BU.print1 ">>> Reached a %s with selective unfolding\n"
                                                 (Print.fv_to_string fv));
          comb_or [
           (match cfg.steps.unfold_only with
            | None -> yes
  