module type Key =sig..end
type t 
val 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 -> t
val sexp_of_t : t -> Sexplib.Sexp.t
val compare : t -> t -> inthash must be non-negative.  An exception will be raised in the
      case that hash returns a negative value.