module type HashedType = sig  endHashtbl.Make.type t 
val equal : t -> t -> boolval hash : t -> intequal, then they have identical hash values
          as computed by hash.
          Examples: suitable (equal, hash) pairs for arbitrary key
          types include
          ((=), Hashtbl.hash) for comparing objects by structure, and
          ((==), Hashtbl.hash) for comparing objects by addresses
          (e.g. for mutable or cyclic keys).