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