module type Elt = Hashtbl.Keytype 
val compare : t -> t -> intval hash : t -> inthash must be non-negative.  An exception will be raised in the
      case that hash returns a negative value.val t_of_sexp : Sexplib.Sexp.t -> tval sexp_of_t : t -> Sexplib.Sexp.thash must be non-negative.  An exception will be raised in the
      case that hash returns a negative value.