anchor.diff:                let t = S.extend_app head (t,aq) None r in
                 rebuild cfg env stack (maybe_simplify cfg.steps t)
   
  -            | Match(env, branches, r) :: stack -> 
  +            | Match(env, branches, r) :: stack ->
                 log cfg  (fun () -> Util.print1 "Rebuilding with match, scrutinee is %s ...\n" (Print.term_to_string t));
                 let norm_and_rebuild_match () =
                   let whnf = List.contains WHNF cfg.steps in
  
positive.code:                 | UnivArgs _::_ ->
                    failwith "Ill-typed term: universes cannot be applied to term abstraction"
  
                 | Match _::_ -> 
                    failwith "Ill-typed term: cannot pattern match an abstraction"
  
                  | Arg(c, _, _)::stack -> 
  
negative.code:                  let stack = us::stack in
                   norm cfg env stack t'
       
            | Tm_fvar f -> 
              let should_delta = match cfg.delta_level with 
                  | NoDelta -> false
  