anchor.diff:                   norm cfg env stack t'
   
             | Tm_fvar f ->
  -            let t0 = t in
               let should_delta =
                   cfg.delta_level |> BU.for_some (function
                       | Env.UnfoldTac
  
positive.code:                     rebuild cfg env stack t
                  end
          end
  
  (* Reifies the lifting of the term [e] of type [t] from computational  *)
  (* effect [m] to computational effect [m'] using lifting data in [env] *)
  
negative.code: 
              if not should_delta
              then rebuild cfg env stack t
              else do_unfold_fv cfg env stack t f
  
            | Tm_bvar x ->
              begin match lookup_bvar env x with
  