signature restricted =
  sig
    type 'a set
    val equal_sets : ''a set -> ''a set -> bool
    val intersection : ''a set -> ''a set -> ''a set
    val make_set : ''a list -> ''a set
    val member : ''a -> ''a set -> bool
    val remove : ''a -> ''a set -> ''a set
    val subset : ''a set -> ''a set -> bool
    val superset : ''a set -> ''a set -> bool
    val union : ''a set -> ''a set -> ''a set
  end;