anchor.diff:    | Constant c -> constant_to_string c
     | Univ u -> "Universe " ^ (P.univ_to_string u)
     | Type_t u -> "Type_t " ^ (P.univ_to_string u)
  +  | Unknown -> "Unknown"
   
   and atom_to_string (a: atom) =
       match a with
  
positive.code:   | Constant of constant
    | Type_t of universe
    | Univ of universe
    // NS:
    // | Refinement of binder * t
    // | Arrow of list binder * comp_t
  
negative.code:     | Tm_delayed (_, _) ->
        failwith "Tm_delayed: Impossible"
  
      | Tm_unknown -> Unknown
  
      | Tm_constant c ->
        Constant (translate_constant c)
  