   item -> set
	val fromList :       item list -> set
	val fromVector :     item vector -> set
	val toList :         set -> item list
	val toVector :       set -> item vector

	val insert :         set * item -> set
	val insertDisjoint : set * item -> set
	val insertWith :     (item -> unit) -> set * item -> set

	val remove :         set * item -> set
	val removeExistent : set * item -> set
	val removeWith :     (item -> unit) -> set * item -> set

	val union :          set * set  -> set
	val unionDisjoint