module type HashedType : sig ... endHashtbl.Make.type t
val equal : t -> t -> boolval hash : t -> intequal, then they must 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).