anchor.diff:      | Tm_delayed (_, _) ->
         failwith "Tm_delayed: Impossible"
   
  -    | Tm_unknown ->
  -      failwith "Tm_unknown: Impossible"
  +    | Tm_unknown -> Unknown
   
       | Tm_constant c ->
         Constant (translate_constant c)
  
positive.code:   | Constant c -> constant_to_string c
    | Univ u -> "Universe " ^ (P.univ_to_string u)
    | Type_t u -> "Type_t " ^ (P.univ_to_string u)
  
  and atom_to_string (a: atom) =
      match a with
  
negative.code:     debug (fun () -> BU.print1 "Readback: %s\n" (t_to_string x));
      match x with
      | Univ u -> failwith "Readback of universes should not occur"
      
      | Unknown -> S.mk Tm_unknown None Range.dummyRange
      
      | Constant Unit -> S.unit_const
      | Constant (Bool true) -> U.exp_true_bool
      | Constant (Bool false) -> U.exp_false_bool
  